Improve command recognition & help text

[emilyhunt]

There are many cases when the bot doesn't quite recognise a command, such as:

• When someone has a newline marker between the mention and the command
• When someone includes punctuation marks
• When someone accidentally adds too many spaces
• If someone just replies to the bot and doesn't mention them (harder to fix)
• Quote posts aren't recognized at all

... and more.

It would be great for the usability of the bot if:

1. The method to turn a post into a set of words to match could be more robust. (Currently this is an ugly thing that should be refactored & improved.)
2. The help text when someone fails to do a command correctly was better.
3. The bot also checked replies for if they contain a command (harder to add but possible, maybe if the first word is correct? I dunno)

[emilyhunt]

Newline & spaces aspects of this were fixed in recent commits. Other stuff is harder, so lowering priority for now...