Create NOOP object which can be returned by a before hook to indicate that the original function shouldn't be called

[monkeyman192]

Currently the original function is always called. This isn't always desirable, and with compound hooks there is currently no easy way around this.
One solution may be to create a singleton object, NOOP (no-op) which, when returned, will indicate to pymhf that the original function shouldn't be called.

The documentation should indicate that this value will have a profound affect on the hook and potentially the entire state of the program being hooked, as a function not returning some value may have large consequences on the game and should only be used with care.