Add `annotations import` and `annotations export` commands

[bradlarsen]

Nosey Parker supports 3 kinds of user-supplied annotations at present:

• Comments associated with findings (the finding_comment table in the datastore)
• Comments associated with matches (the match_comment table in the datastore)
• Statuses (one of ${accept,reject}$) associated with matches (the match_status table in the datastore)

(Note that the open-source version of Nosey Parker provides no mechanisms at present for populating these annotations.)

This pull request adds two new commands to Nosey Parker:

• The annotations export command dumps the annotations from a datastore into a JSON format
• The annotations import command takes a JSON file of annotations and attempts to import them into a datastore

These commands are experimental and likely to change in the future.