Handle inline sourcemaps

[IvanSanchez]

The current logic uses sander to read a filename whenever the sourceMappingURL= regexp matches.

However, that regexp also matches inline sourcemaps like:

/*# sourceMappingURL=data:application/json;base64,aaaaaaaaaaaaaaaaaaaa== */

There is a need for parsing data: URLs (base64-decode them, then JSON.parse() them).

In particular, the gobble-postcss transform generates inline sourcemaps by default, which are a problem if they are concatenated later.