Use `fix-whitespace.yaml` as filter when file arguments are given

[andreasabel]

Use case:

git ls-tree -r master --name-only | xargs fix-whitespace --config fix-whitespace.yaml 

or simply

git ls-tree -r master --name-only | xargs fix-whitespace

This should filter the FILES given to fix-whitespace through the configuration given in fix-whitespace.yaml.

A new flag, like --config - could be used if really all of the given FILES should be processed.