Multiple files support

[ai]

Right now plugin support only single files — just move variables to top.

But in most of cases we need to support multiple files and create some external config file.

Maybe we need to split plugin into 2 plugins: one will return a JSON with variables to callback option (default behcaviour will be save this JSON to CSS variables). Second will take variables list and replace them in files.

Also with 2 plugins system will be very useful for CI — we can run second plugin automatically, but first plugin anyway require manual work.