Add Twitter hashtags as tags in ThreatKB

[rshipp]

When importing a C2 hit to ThreatKB from Twitter, include hashtags as tags.

This requires the following changes:

• [ ] Add tags member to Artifact model
• [ ] Extract hashtags (tweet['entities']['hashtags']) from tweets in Twitter source plugin
• [ ] Push artifact.tags as tags to ThreatKB from ThreatKB operator plugin

[rshipp]

Likely to cause more problems than it solves. Closing.