Allow when & verify to disregard arity

[searls]

#33 implements expected arg length matches actual invocation, which should have been implemented all along (oversight).

This means we should probably provide a way to stub or verify an interaction when we don't care about all the args to work around this. Whatever we do ought to be symmetrical between when & verify's API for ease of understanding.

Some ideas:

drive = td.create()

when(drive(), {ignoreAdditionalArgs: true}).thenReturn("💩")

drive() // "💩"
drive(1,3,4,5) // "💩"

So basically, the second argument to when could be an options object that's stored with the stubbing (or used by the verify) to modify how matching occurs. This gives us some breathing room for other features that would deviate from the default behavior, too.

One more example:

drive = td.create()

when(drive(60), {ignoreAdditionalArgs: true}).thenReturn("💩")

drive() // undefined
drive(60,1,3,4,5) // "💩"
drive(59,1,3,4,5) // undefined