allow filtering for paths

for our tests we need an extended root path,

projectA
  - public
    - foo.js ( requires code from projectB.shared.js)
  - test
     - buster.js (rootPath = '../../', sources: 'projectA/public/*.js' )

projectB
  - public
     - shared.js

in order to run buster the rootPath must be the directory where both projects reside, but that breaks coverage, so i must be able to strip out the "projectA" part of the test sources.

this is possible with this commit, hopefully not breaking existing code
please review and merge, or adjust