`readonly` parameter on `storage.find_game` and `LRRbot.get_current_game`.

`readonly` defaults to `True`.
Merge remote-tracking branch 'andreasots/variable-mutability'
Make return from findgame immutable if readonly requested

For easier failure checking
Add !explain show command
Refresh game whenever the show changes

When the show changes, chances are the game has changed too...
Allow (approximate) plurals for eg !totaldeaths

!totaldeath is still the canonical command, and this is incomplete (not every stat can necessarily be pluralised by adding s) so undocumented... but will hopefully be useful for typos in the chat.

Having it be plural just feels more right, anyway, so understandable that people would do that.

This commit message is now 206 times longer than the actual change.
No reason this can't use the annotation pattern