Added `liftMaybe` for lifting Maybe into MonadError

[emlautarom1]

On #37 it was discussed the existence of liftMaybe:

    I think `lift` is fine, but would also like `liftMaybe`.

Originally posted by @ocharles in https://github.com/haskell/mtl/issues/37#issuecomment-325640686

This PR adds liftMaybe, also called note in some custom preludes like Universum and Protolude

On Universum, they suggest using maybeToRight, which involves liftMaybe e = liftEither . maybeToRight e. Personally, I'm not a big fan of such boilerplate.

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Lifts a Maybe into any MonadError e, using a supplied e as the error if the Maybe is Nothing.

do { val <- liftMaybe e =<< action1; action2 }

where action1 returns a Maybe.