Feature/add datasources contribs diff stix

[IanDavila]

PR to update diff-stix.py script to include data sources and contributors section when selecting markdown option.

New contributors are calculated as defined in #71 and script was tested for If a contributor is removed from an object and (not re-added on a different object).

data["new_contributors"] will contain a dictionary of keys (names of contributors) and values (number of new contributions for the release) after the load_data() functions runs.

PR also adds new argument --use-mitre-cti that downloads the MITRE CTI bundles specified by the domains that may be used for the old comparison.

Closes #70, #71.