Summary: Add "bors cancel" as alias for "bors r-"

I allow this RFC document to be modified and redistributed under the terms of the Apache-2.0 license, or the CC-BY-NC-SA 3.0 license, at your option.

Motivation

It's just readability of the chat history.

Guide-level explanation

Use bors cancel to cancel a PR from the queue.

command	description
bors cancel	Equivalent to bors r-

We already have one synonym. There's not much reason not to have two.

Custom commands? No way. It's hard enough making sure everyone understands what's going on.

What's the cut-off? We don't want to enable every possible one. cancel seems alright, because other parts of the app call it "canceling," but if someone proposes "stop," we might want to say no.

Since we already refer to it as "cancel"ing in other places, this makes sense, but beyond that, there hopefully won't be any more of these.