Add 'root' option and 'urlTransform' option

[gauntface]

CSS imports can be absolute paths on a web server where '/styles/...' could be a valid server path, but relate to a different path on the file systme (i.e. '/public/styles/...'). Would you be open to adding the following options to solve this use case:

• root: A path that is pre-pended to absolute paths used
• urlTransform: a function that is called with each discovered importPath and can return an alternative import path (allows developers to use a development version of a file while working on the source but switch to an optimsed version later on).

[unlight]

@gauntface
Seems that urlTransform more general approach and overlaps root feature.
Why do we need both?

[gauntface]

Agreed, root would just cover the most common use case and urlTransform is the more involved option

[unlight]

Implemented transform option in v5.1.x