Add automated testing infrastructure

[super-cooper]

Is your feature request related to a problem? Please describe. We keep running into simple bugs because we have no unit tests or anything of the sort. We should have a series of tests that are required to pass before a change is merged.

Describe the solution you'd like For now, something simple like the python unittest framework should suffice. If our needs become more complex, we can use something more robust later.

There is also a library specifically made for testing Discord bots that @nhawke has mentioned to me before. We should absolutely incorporate this into our unit tests.

Describe alternatives you've considered The only other Python unit testing framework I know of is PyTest, but I've never used that before.

Additional context We should probably use continuous integration to prevent things from being merged without testing. This should be a separate, future ticket that depends on this one.

[nhawke]

Using the discord bot testing library probably isn't necessary. This will be easier with #19 done.

[super-cooper]

We should do it anyway so we can test what output looks like

[super-cooper]

https://github.com/JakeCover/distest

[super-cooper]

http://mypy-lang.org/

[super-cooper]

Re-assigning this to myself since it's blocking a bunch of stuff and I feel like working on the bot