Initial support for detailed diff of changes.

[nickamon]

Initial commit of beta code for supporting detailed diff of ATT&CK changes. I envision there will be additional refactoring required but wanted to give early access to the code in order to facilitate feedback.

Example invocation:

python3 diff_stix.py  -types technique -domains enterprise-attack -markdown --no-details x_mitre_platforms --no-details x_mitre_data_sources --no-details external_references --no-details x_mitre_contributors

@isaisabel