Explore using license text files to build text matchers for license insertion

Currently the text matchers defined for standard licenses use inline text copied from license files.  This change would allow a file to be specified in the matcher definition and read the contents of the file as the text to match.

In addition, currently the insert header only inserts Apache v.2 licenses.  This change should explore being able to identify a custom license to insert (e.g. the license file)