plugins: Add crosslink plugin

[jayvdb]

Reviewers Checklist

• [ ] Appropriate logging is done.
• [ ] Appropriate error responses.
• [ ] Handle every possible exception.
• [ ] Make sure there is a docstring in the command functions. Hint: Lookout for botcmd and re_botcmd decorators.
• [ ] See that 100% coverage is there.
• [ ] See to it that mocking is not done where it is not necessary.