Controllable auto-close/postpone behavior

[Centril]

See discussion on #197 and #92.

Possible control mechanism in the teams.toml file (renamed to rfcbot.toml):

[fcp_behaviour."rust-lang/rfcs"]
close = true
postpone = true

[teams]

# other stuff....

[anp]

This style looks good to me!

[anp]

To answer your question from the PR:

One thing I'm a bit unsure about as to making this work for all repos is whether there is one instance of the rfcbot program running for rust-lang/rust and rust-lang/rfcs or if there are separate instances for each.

There's only one instance of rfcbot running for all repos it watches at the moment, so I think we'll need the behavior to be configurable.