Sourcemap tracker fails when input sourcemap is data URI (inline)

[cspotcode]

In "input-source-map-tracker.js", it automatically locates the /*#sourceMappingURL= in the input .css file (very cool!) but does not account for the possibility that the URL is a data URI:

/* for example, an inline sourcemap generated by Stylus */
/*# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJzb3VyY2VzIjpbImF..... */

https://github.com/jakubpawlowicz/clean-css/blob/master/lib/utils/input-source-map-tracker.js#L71-L73

It attempts to read this file off disc:
/path/to/source/data:application;base64,eyJ2ZXJzaW9uIjozLCJzb3VyY2VzIjpbImF.....
which throws an ENAMETOOLONG error.

I think a fix is fairly simple. Add a dependency on a data URI parsing library. Check the sourceMapFile against a data URI regexp, perhaps /^data:/. If it matches the regexp, parse it as a data URI. Otherwise keep the current behavior: reading a file off disc.

[cspotcode]

I wrote a patch that implements the fix:
#600

I didn't write any tests, though. If you want to merge this, I can look into making some tests.