filters.py: Filter required users

[Vamshi99]

If RESP_ONLY_REQ_USERS configuration is true, then stop responding to users other than those in the REQUIRED_USERS list in config.

Closes https://github.com/coala/corobo/issues/515

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.