Add ability for a Slack user to list available groupings they can join

[cpruitt]

See PR: #30

We'd like the ability to allow a Slack user to ask for a list of slack channels they can join. Something like a slash command:

doubleup list

This could list out all of the configured slack channels. We can likely pull descriptions from the channel API.

Do we need to limit visibility of channels for any reason or is are channel access controls enough of a security measure?

[cpruitt]

Stalled because of time. Work in progress in #30