Move Slack-specific slackcat logic in to a separate package

[thisisaaronland]

We did this mostly so that we could do this:

https://github.com/whosonfirst/go-slackcat-writer (specifically so we could use it with io.MultiWriter for logging)

Which could have been done without using slackcat or just copy/pasting the relevant code but since we like slackcat this seemed like a better idea.

The Makefile stuff is mostly because Go build/packaging still feels like a weird chicken-and-egg problem, but maybe I am just doing it wrong?