shorthand single command both to change topic and logging location

[dwsinger]

it seems it's easy to forget to change topic: before github: or vice versa.

for some, it might be that a simple
gh:
start logging to that issue, and stops logging to a previous issue (if any)
gh: none
or
gh:
stops logging altogether

would suffice

[dbaron]

This is similar to #45 (which I'd like to do) and #50 (which I'm less enthusiastic about given early experience when I first had the CSS WG use the bot in our Tokyo meeting; see my comment there).

I just spent substantially more time than I intended updating the dependent libraries that the bot depends on (which did let me pull in a bug fix I wanted, and should also generally allow code cleanup and more modern Rust when I get around to it), so I probably need a bit of a break from bot work for a bit, though.