Add exception support to no-mutation rule

[scottnonnenberg]

By adding a { object: 'module', property: 'exports' } exception, fixes #6.
By adding a { property: 'propTypes' } exception, fixes #14.

You can now provide an exceptions array in the options for no-mutation in your eslint config:

{
  'no-mutation': ['error', {
    exceptions: [{
      object: 'module',
      property: 'exports',
    }]
  }]
}

Also adds tests with 100% code coverage to the project. :0)

[jfmengels]

Seeing as there is no maintenance of the plugin, I've written my own that has this exception system.

[scottnonnenberg]

I've also got a plugin available with these exceptions. :0) It has a little bit of extra code to special-case 'this', which you might want to pull into your plugin as well: https://github.com/scottnonnenberg/eslint-plugin-thehelp/blob/master/src/rules/no_mutation.js

[jfmengels]

Hmm... Yeah. For an app written in a FP way, the use this is strongly discouraged, but for a React app, it might be useful. I'll add the exception behind a flag, good idea. Thanks :)