Add a new option importPathFormatter to format the path of css files

[xile611]

In my case, I need to update of path of css files after transforming the css-modules.
So I add a new option importPathFormatter.
But It seems the test cases fail in my local environment which also fails in mater branch.

Test error messages:

 1) should write a multiple css files using import
    2) should write a multiple css files using import preserving directory structure
    3) should write a multiple css files using require
    4) should write a single css file using import
    5) should write a single css file using require
    6) should extract styles with a single input file via gulp
    7) should extract multiple files via gulp
    8) should extract combined files via gulp
    9) should call custom preprocess

....

 1) babel-plugin-css-modules-transform should write a multiple css files using import:
     Error: ENOENT: no such file or directory, open .../babel-plugin-css-modules-transform/test/output/parent.css'

[michalkvasnicak]

Released as 1.6.0

[michalkvasnicak]

Thank you 😉