use newer features of Slack's slash command integration

[tmaybe]

Slack released some enhancements to its slash command integration a few months ago. You can now:

• style private messages
• respond publicly in a channel without requiring the incoming webhook integration

I definitely like the ability to style private messages and will add that. It may also fix the unclickable links bug.

A public response from a slash command makes you echo the command you typed publicly as well, which I think is uglier than the straightforward incoming webhook, so I may not change the way I'm doing it now.