Fix URL and Relative Windows file path support #235/#239

Address all of #235 and the portion of #239 dealing with URLs including a properly formed `file://` schema. 

Based on the nature of this behavior, I'm not sure if any clear way to add tests to verify this behavior to make the verification more robust. There is currently a failing test in `common/config_test.go` when comparing the file URL generated against a custom built URL string because of the mismatch of the Windows file path separators.

/cc @rasa