Hand over the matched option value into the autocomplete callback

[codemasher]

This saves digging through the entire Interaction instance again in order to find the focused value that triggered the callback.

[codemasher]

I'm taking out the thoughts from the OP and leave them here, this is material for another day as there are currently too many open questions re this:

I'd also like to hand over the matched command/subcommand/subcommandgroup, for which I'd like to dissolve that recursive call (recursions are a bit overused in your library anyway for my taste :D). From what I gather, the options instance is nested similar to the description here, or is there something I'm overlooking?
3 nested foreach calls might look a bit clunky, but are probably faster and leave the option to hand over the command names too.

re OP: LGTM?