Team targeted fcp merge/cancel/postpone

[Centril]

In a conversation, @nikomatsakis said:

(I wish that you could specify the team(s) to rfcbot as part of the command)

Seems reasonable to me; we could reuse the mechanism for @rfcbot poll in the command parser.

[dwijnand]

Mentioned also in #92, where @nrc says:

Adding style to the rfcbot command feels worse than having a t-style label on everything (but not much)