Add static analysis (credo and dialyzer) and formatter

[ryanwinchester]

• [x] Added credo.
• [x] Added dialyxir.
• [x] Used formatter.
• [x] Added them to CI.
• [x] Removed travis config since it's broken and there are Github Actions.
• [x] Updated comments for consistency (like adding periods, since a lot had them but a lot also didn't).
• [x] Added a few more typespecs.
• [x] Updated deps.

Not sure your opinion of some of these tools, let me know.

Action is running on fork: https://github.com/ryanwinchester/exirc/actions/workflows/tests.yaml