[base] use env flags to config severity level

[hshoff]

Best viewed ignoring whitespace: https://github.com/airbnb/javascript/pull/1037/files?w=1
addressing: #1015

⚠️ WIP... just looking for implementation feedback.

Currently it just reads from process.argv and parses out severity level flags. Flag options: --warn, --off, --error.

Usage would be from the command line like --warn=some-rule,another-rule --off=semi

Not positive if this is the best way to control severity levels. Would prefer to have this as a field in .eslintrc called overrides or something. Open to other ideas.

before:

$ node
> require('./best-practices')
    rules: {
      'accessor-pairs':  'off',
      'array-callback-return': 'error',
      'block-scoped-var':  'error',
// ...

after:

$ node
> process.argv.push('', "--warn=accessor-pairs", "--off=block-scoped-var")
4
> require('./best-practices')
{ rules:
   { 'accessor-pairs': 'warn',
     'array-callback-return': 'error',
     'block-scoped-var': 'off',
// ...

@ljharb @lencioni

[ljharb]

I'm not sure if this is the approach we want.

Primarily, I don't think we want per-rule granularity - override comments are best for those. I'm going to close this; but let's discuss this in person.

[hshoff]

👍