Allow unspecified values for composite types

[Kamekameha]

With this, behavior is (or should be) similar to when only nargs is used. The Tuple parameter type can't actually handle empty tuples and thus we get the TypeError exception about differing value length and type arity.

Hopefully this fixes #472 but I'm not sure if this is the correct approach into fixing this since composite parameter types aren't exactly required to return tuples. Besides, ideally, for nargs unequal to 1 nargs parameters should be designed on top of the Tuple parameter type, in my opinion, and like any other type without defaults, composite types should return None when unspecified in the command line (unless multiple is True, which, as is already the case, should return an empty tuple). Of course that's a way different approach and I'd rather discuss that in here first.

I'm definitely open to suggestions or something as I'm not really familiar with click's internals. All tests runs passed for me so I hope I didn't break anything.

(Also, last commit can be ignored if necessary, I just found it appropriate in this case.)

[dsully]

@Kamekameha Thanks for the PR. Could you add some inline comments to your changes, and perhaps some documentation?

Thanks!