feat: introduce Plugins API, fix tests

[tunnckoCore]

• fix silent failing tests (test/unit/test-incoming-form.js)
• introduce Plugins API
• each parser to accept options in constructor
  • write them to its this.globalOptions = { ...options };
• close Allow passing custom (querystring) parsing module(s) #391
• close respect form hash option on incoming octect/stream requests, enabling… #407
• close Fields with brackets ([]) #386
• close do not create empty temporary file if file not selected #374
• close handle case when file not selected - continue of #374 #521
• close Added support for uploading files to buffer instead of filesystem #267
• close Better tests and code organization #415
• related form type 'x-mixed-replace' #541
• related Support parsing of multipart/byteranges #390

Why?

Better separation. Easier future testing. More extensible so there can be developed third-party plugins (for example more advanced QueryString parser). We now can unit test each separate plugin/parser.

Plugins API

add form.use(plugin: Plugin) method

A method that allows you to extend the Formidable library. By default we include 4 plugins,
which esentially are adapters to plug the different built-in parsers.

The plugins added by this method are always enabled.

See src/plugins/ for more detailed look on default plugins.

The plugin param has such signature:

function(formidable: Formidable, options: Options): void;

The architecture is simple. The plugin is a function that is passed with
the Formidable instance (the form across the README examples) and the options.

Note: the plugin function's this context is also the same instance.

const formidable = require('formidable');
const form = formidable({ keepExtensions: true });
form.use((self, options) => {
  // self === this === form
  console.log('woohoo, custom plugin');
  // do your stuff; check `src/plugins` for inspiration
});
form.parse(req, (error, fields, files) => {
  console.log('done!');
});

Important to note, is that inside plugin this.options, self.options and options
MAY or MAY NOT be the same. General best practice is to always use the this, so you can
later test your plugin independently and more easily.

If you want to disable some parsing capabilities of Formidable, you can disable the plugin
which corresponds to the parser. For example, if you want to disable multipart parsing
(so the src/parsers/Multipart.js which is used in src/plugins/multipart.js), then you can remove it from
the options.enabledPlugins, like so

const { Formidable } = require('formidable');
const form = new Formidable({
  hash: 'sha1',
  enabledPlugins: ['octetstream', 'querystring', 'json'],
});

Be aware that the order MAY be important too. The names corresponds 1:1
to files in src/plugins/ folder.

Pull requests for new built-in plugins MAY be accepted - for example,
more advanced querystring parser. Add your plugin as a new file
in src/plugins/ folder (lowercased) and follow how the other plugins are made.

[GrosSacASac]

Nice

[tunnckoCore]

Not bad.

-----------------|---------|----------|---------|---------|---------------------------------------------------
File             | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s                                 
-----------------|---------|----------|---------|---------|---------------------------------------------------
All files        |    87.6 |    78.38 |   85.87 |   87.66 |                                                   
 src             |   83.74 |    70.71 |   87.23 |   83.54 |                                                   
  File.js        |   89.47 |       75 |    87.5 |   89.47 | 50,56,65,66                                       
  Formidable.js  |   82.09 |    70.11 |   86.84 |   81.91 | ...76,304,309,337,338,352,353,365,375,376,379,407 
  index.js       |     100 |      100 |     100 |     100 |                                                   
 src/parsers     |   89.34 |    82.41 |   81.82 |   89.67 |                                                   
  Dummy.js       |    62.5 |        0 |      50 |    62.5 | 15-17                                             
  JSON.js        |    87.5 |        0 |     100 |    87.5 | 27,28                                             
  Multipart.js   |   91.75 |    84.62 |   83.33 |   92.19 | ...57,188,197,216,222,262,282,285,309,326,333-335 
  OctetStream.js |     100 |        0 |     100 |     100 | 6                                                 
  Querystring.js |   68.75 |      100 |   66.67 |   68.75 | 26,30,31,36,37                                    
  index.js       |     100 |      100 |     100 |     100 |                                                   
 src/plugins     |   91.37 |    84.62 |   86.96 |   91.37 |                                                   
  index.js       |     100 |      100 |     100 |     100 |                                                   
  json.js        |     100 |      100 |     100 |     100 |                                                   
  multipart.js   |    98.7 |     88.1 |     100 |    98.7 | 24                                                
  octetstream.js |   93.94 |    66.67 |     100 |   93.94 | 54,73                                             
  querystring.js |      25 |       50 |      25 |      25 | 16,24,26,28,29,32-34,37                           
-----------------|---------|----------|---------|---------|---------------------------------------------------

=============================== Coverage summary ===============================
Statements   : 87.6% ( 551/629 )
Branches     : 78.38% ( 203/259 )
Functions    : 85.87% ( 79/92 )
Lines        : 87.66% ( 547/624 )

[GrosSacASac]

Thank you for adding me as a contributor

[tunnckoCore]

@GrosSacASac, Thank you for adding me as a contributor

Sure, of course ; )

---

trying to sync the bot and GH actions... ;d

@all-contributors please add @tunnckoCore for idea and doc

[tunnckoCore]

Happy birthday to me 🎉

Available through formidable@2.0.0-dev.20200130.1 or formidable@dev

[tunnckoCore]

@GrosSacASac , it's generally ready. There are two test suites - one is the old, second is with Jest (the test/unit). Both are passing, but we need to continue converting them. But that could be after v2.

After merging this a lot will be closed too, so v2 is really near, just need to add a few easy fixes and features like #154 and #153

I hope you are okay with the ton of stuff updates on the master and this PR.
I really like the whole thing now. 🎉 ❤️ I even have one more idea in the pocket.

[tunnckoCore]

The only thing I'm wondering is when and why the performance dropped to 1500mb/sec, it was up to 2500mb/sec for some time. And I didn't touched the multipart parser... Hm... 🤔