[remind] [minor] Add backreference functionality to modules/calc.py

[gkistler]

Allow for the following:

<User> .c 5 + 5
<pyBurlyBot> 5 + 5 = 10
<User> .c * 500
<pyBurlyBot> 10 * 500 = 5,000

Some quick thoughts:

• Before this was per-channel, but it might be a better idea to make this per-user instead.
• Not really critical to persist this stuff to the database, this would probably be a good fit for a state helper.

[Clam-]

Regarding state, I wasn't overly keen on using state since it seems kind of useless and waste of overheads, but it certainly could have a dictionary or something for modules to store some data in. Maybe there should be an API for modules to have access to in-memory key/value store thing. I suppose modules could use their own dictionary or something.

[Clam-]

I use database to store last seen youtube URL and will use it for last seen normal URL so I don't think it would be terrible to use it for last user calculation either.

[Clam-]

Current implementation of calc makes this very unlikely to happen.