CLI: Return code and CSS modules as JSON when output file is not given

[joelmoss]

My very first rust experience, so be gentle ;)

I wanted a way to get both the resulting code and the CSS modules from the CLI without having to specify an output file, which means I would have to read files to get what I want.

So in this PR, if output-file flag is not given, and css-modules flag is given, the CLI will return a JSON string containing the code and the CSS modules...

{
  "code": ".cLD2Uq_header {\n  background-color: #00f;\n}\n",
  "modules": {
    "header": { "composes": [], "isReferenced": false, "name": "cLD2Uq_header" }
  }
}

All other functionality is maintained.

Would appreciate any feedback. thx

[devongovett]

Technically, this is a breaking change but I think we can get away with it.

[devongovett]

We should probably update the tests for this.

[devongovett]

FYI added tests for this and your other PR in a5fdcc1. Also renamed "module" to "exports" in the output JSON to match the JS API.