feat: support multipart/form-data

[erunion]

🧰 What's being changed?

• Adds general support for multipart/form-data requests and the ability to accept file uploads.
• Updates SDK snippets to comply with these changes.

In order to upload a file as part of a multipart/form-data request (or one of its supported variants), you can just give a path to the file as the value of the property in the parameters for the request. For example:

sdk.post('/anything', { documentFile: 'owlbert.png' })
  .then(res => res.json())
  .then(res => {
    console.log(res);
  });

This will automatically locate owlbert.png, read it into memory, convert it into data URL, and hand it off to fetch-har.

You can also give it a stream, and the SDK will handle everything as well like magic.

sdk.post('/anything', { documentFile: fs.createReadStream('owlbert.png') })
  .then(res => res.json())
  .then(res => {
    console.log(res);
  });

Resolves #131 and #130

[domharrington]

💪

[erunion]

Holding off on merging this in until next week because after running some tests against our own API, fetch-har still needs some work.

{ error: 'SPEC_INVALID',
  message:
   'The spec you uploaded isn\'t a valid JSON or YAML file. Here\'s the error we found: Not valid YAML',
  suggestion:
   'Try running your spec through a validator to see if they pick up any issues with the formatting!',
  docs:
   'https://docs.readme.com/developers/logs/********',
  help:
   'If you need help, email support@readme.io and include the following link to your API log: \'https://docs.readme.com/developers/logs/********\'.',
  poem:
   [ 'When we asked for your swagger',
     'It seems you misunderstood',
     'You sent us a cool attitude',
     'Instead some JSON would have been good' ] }

Turns out sending a data URL instead of file contents doesn't actually work in real-world scenarios.