Add an option to the rbt diff command to 

apply basic colorization to the output.

The rbt diff --color-diff command highlights 

removed lines in red and added lines in green.

This review request includes the following:

Utilizing pygments library, format the diff displayed 

on terminal with basic colorization.
Add --color-diff option to rbt diff command.

Project documentation and notes

Created test_diff.py that contains test cases for rbt diff command
Tested command arguments and formating the diff using pygments library.