Add support to filter the assets that should be precompiled

[lucasmazza]

Most of the times you would just precompile a single js or css file, instead of the entire project.

[]'s

[maccman]

Yes, this is a tricky one. So your change means you can run: build app/assets/application.js ?

[lucasmazza]

No, just build application.js application.css

[maccman]

Cool. I've added you as a contributor on the repo, but let's still do changes as a pull request. Thanks so much!

[lucasmazza]

Thanks!