Allow user to post reminder in a different room

[kb0rg]

This issue still needs some design discussion, as well as user feedback.

The remind command has the ability to specify a "target" to be mentioned for the reminder (e.g. me, room, here). It does not, however, allow reminding in a room other than this one—particularly useful if one wanted to set up a reminder for a room from a DM with Valkyrie.

Following the pattern of the slack remind command, the room name might be passed as one of the "who" options in our remind syntax, eg:
\remind [me|team|here|flowname] <what> <when>

However, we may want to re-evaluate that pattern, since someone might want to notify in a different room, but also decide whether or not to tag a specific user or the flow's team.