Add package.json fields configuration to usage in cli

[kareniel]

Configuring Bankai is done by modifying package.json. It'd be cool if you could get the usage info for the browserify, sheetify, and documentify fields in the usage text (--help) in the CLI.

Possible approach

Not sure about the wording, but:

1. Add this line to usage:

For configuration usage, type `bankai --help config`.

2. Output something like this:

Configuration


Bankai is built on top of compilers for scripts, styles and documents. 
Each of them can be configured by adding a field to your project's package.json.

These three fields are, respectively: "browserify", "sheetify" and "documentify".
Each one should have a configuration object as it's value.

There is currently one possible configuration field: "transform".

It can be one of either:

1. An array of transform names. 
       ie:  [ "babelify" ]
2. An array of tuples transform name + configuration object. 
       ie: [[ "babelify", { "presets": [ "env" ] } ]]

Full example:

{
  "browserify": {
    "transform": [
      [ "babelify", { "presets": [ "env" ] } ]
    ]
  },
  "sheetify": {
    "transform": [
      "sheetify-cssnext"
    ]
  },
  "documentify": {
    "transform": [
      [ "posthtmlify", { "use": [ "posthtml-custom-elements" ] } ]
    ]
  }
}

3. Profit

[yoshuawuyts]

@kareniel that's a great idea!

[kareniel]

Closed by #437 :)