label names handled by the shortcut command should be configurable

[JohnTitor]

Currently the label names handled by the shortcut command are hardcoded:

triagebot/src/handlers/shortcut.rs

Lines 29 to 31 in a2ae093

	let waiting_on_review = "S-waiting-on-review";
	let waiting_on_author = "S-waiting-on-author";
	let blocked = "S-blocked";

That would enforce other repos to use the exact same names, but that restriction shouldn't be necessary and we should be able to change them as we want.

[Mark-Simulacrum]

Happy to see a PR changing this, so long as we preserve the defaults (at least until rust-lang/rust updates its configuration with the right labels).