Generate a manifest json of export tokens

[joshwnj]

This adds an optional json config (or --json via CLI). If provided, all export tokens are written to a json file (indexed by source css filename).

I haven't tried using this but wanted to see if people are interested in trying it. Could potentially open up a way to use css-modules with non-js backends.

[joshgillies]

Nice! Might be worth adding an "advanced usage" section to the ReadMe, outlining where this would be useful?

[joshwnj]

Yeah, or like a "Using css-modules on the backend". We can mention the require hook for people using node, and json manifest for others.

[wbyoung]

For what it's worth, it took me finding this issue to understand better the comment on the README:

Option B: configure the jsonOutput option with a file path and css-modulesify will generate a JSON manifest of classnames.

I was confused because writing a manifest is helpful in a sense, but still means that whatever JS code you have can't be used (without some alteration) for both the front end and the backend.

Also, in case you're interested (since the PostCSS community still seems to be new & growing), I've written a babel transform that can handle CSS modules and all async PostCSS plugins that I'm using with Browserify for the frontend and transplied for the backend.