parse ¬ (\neg) as a prefix operator

As [discussed on julia-users](https://groups.google.com/d/msg/julia-users/oqdUhGFXVbw/3KgWmpfWY6oJ), this patch parses `¬` as a prefix operator like `!`, although it doesn't define it by default (since `!` is already perfectly good).

The main arguments for this, in my mind, are:

* There is nothing sensible to do with ¬ except to parse it as a prefix operator.
* We already parse the other logic symbols (`∧`) as operators, so we might as well complete the set.

@jverzani also has a use-case in SymPy.