privmsgFromPlugin: Rewrite using MaybeT monad transformer

OMG it works!!! This took me a week of off-and-on tinkering, learning,
procrastinating, and struggling. So awesome that it's finally working!

The `liftMaybe` was key, learned that from:
https://stackoverflow.com/questions/8684252/how-to-inject-a-maybe-value-into-maybet

This article on monad transformers by 'kqr' also helped quite a bit:
https://github.com/kqr/gists/blob/master/articles/gentle-introduction-monad-transformers.md

A Wikibooks article on the same was somewhat useful:
https://en.wikibooks.org/wiki/Haskell/Monad_transformers

And this was an interesting look at Alternatives, which I didn't end up
using but was interesting nonetheless:
http://www.parsonsmatt.org/2016/11/18/clean_alternatives_with_maybet.html

When I learned about monad transformers I thought: "Yes! This is what I
need to clean up the `privmsgFromPlugin` function.". I wasn't liking the
nested `case` statements there, and since everything was a `Maybe`, I
thought, why can't we chain the `Maybe`s like we're supposed to be able
to with monads? Well, turns out we can. It just involved a lot of tricky
`lift`ing. This gets rid of the nesting, resulting in a much cleaner-
looking function. So cool!

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