Add a timezone option for commands

[seeroflights]

Looks like it might be a different approach from #161. For this you wouldn't have to cache everything though? Not sure how everything works, this might not be feasible.

Anyways, it would be nice to have a timezone option on commands like /heatmap and /rate. Use cases I can think of off the top of my head:

• Running commands in DMs would have the correct timezone offset
• People with long usernames don't have to include their timezone in their nick to get the right offset
• You can run commands for other people and have it in the right timezone, e.g. /heatmap user:Cloakknight timezone:-7