Time: refactor code, make tztime respect plugins.Time.format

In `tztime`:
- Import pytz on load, not every time the command is called.
- Respect `plugins.time.format`.
- Use `irc.error(e, Raise=True)` instead of `return` in error handling.
- Fix a typo in the command description.

In `time`:
- simplify the code used for getting the format configuration (the if statement doesn't appear necessary and the option can just be fetched using `msg.args[0]`).