Feature: Use slack buttons for configuration

[jfairley]

Rather than issuing command keywords (channel foo or add snippet bar), offer message buttons with the "value" to follow as a plain comment.

1. What would you like to do?
   • Slack Channel
   • Add Snippet
   • Done
2. click Slack Channel
3. type foo, hit Enter

reference: https://api.slack.com/docs/message-buttons