Make Option work with Optionals

[mdiep]

In Carthage/Carthage#1143, it would be nice to use an Option<String?> for a command-line option. Since the default value isn't really known, it would be preferable to default to nil. This PR makes that possible, but it introduces a breaking change.

• Make Option require a defaultValue. Previously, a nil default meant that the parameter was required. Carthage doesn't use this, and I think that behavior is a little silly given that it's called option. If this behavior is required somewhere, I'd rather see another type added.
• Make a version of <| that works with an Option<T?> where T: ArgumentType.

The implementation isn't as nice as I'd like. I ran into Swift limitations, segfaults, and, I think, some type inference errors. But ultimately I arrived at something that at least works.

[ikesyo]

I will take a look in a couple of days.

[ikesyo]

Or, the non-Optional version of <| would call the Optional version. Unfortunately, those don't work.

I tried an implementation: 715684d. How do you feel about it?

[ikesyo]

Other than that change, LGTM 👍