partial port of Python difflib package to Go

 Go-difflib is an as yet partial port of python 3's difflib package.
 Its main goal was to make unified and context diff available in pure Go,
 mostly for testing purposes.
 .
 The previous owner of this project (pmezard) did not have the time to
 continue working on it (github.com/pmezard/go-difflib), so ianbruene
 continues to develop it at github.com/ianbruene/go-difflib.