Add support for regex in root_sass

[DoobleD]

Hi,

Thanks for providing this bundle!

Currently we need to manually add every root sass files that we write under the symfonycasts_sass.root_sass config.

Usually non root files are prefixed with _, while root file aren't. It'd be great to have symfonycasts_sass.root_sass support regular expressions.

That way we could simply set one line there that automatically lists all root files and forget about it. It would also avoid errors when we forgot to add the entry and the unlisted scss file is then silently mapped to a css file that actually still contains sass.

This is somewhat linked to #41 which attempts to make clearer what files are being built.