Add flipped infix withDefault (?) operator from Maybe.Extra (aka the existential operator)

[rehno-lindeque]

Following @evancz's comments here https://github.com/circuithub/elm-array-extra/pull/1#issuecomment-88966985 I thought there should be an issue here for discussion.

The question that remains appears to be what precedence the ? operator should have. Here's how it is defined in Maybe.Extra at the moment:

{-| Flipped, infix version of `withDefault`.
    head [] ? 0 == 0
-}
(?) : Maybe a -> a -> a
mx ? x = withDefault x mx

Anecdotaly, the default operator precedence for new operators has worked pretty well for me so far if that's any help...

[evancz]

Let's move this to #216

[evancz]

Let's move this to #216