Minecraft→Slack chat syncing

[fenhl]

(See also #7.)

Basic functionality would be to forward all chat messages from the main world to #general. To do this, we need to run logtail in one thread, and the Slack handler in another. There are two options I can see:

1. Rewrite Slack integration using the slack_api crate.
2. Wait until Allow async message sending slack-rs/slack-rs#50 is fixed.

Opinions?

[fenhl]

The upstream issue is fixed, so this should be fairly easy now. We just need to filter chat messages from the rest of the log, like the original wurstminebot did.