Add ability to get applicable markings for a STIX construct

[bworrell]

There should be some mechanism by which a user can determine the applicable markings for a given construct. This could be a get_markings() method associated with each stix.Entity implementation, or a global get_markings(...) or is_marked(...) method.

[emmanvg]

Super late, but we have released https://github.com/mitre/stixmarx and it provides such ability. Unless there is further discussion on the topic, I am happy with closing this issue.

[gtback]

Yep, I'd say we can close this!