Add support for factories to `Optional`

[todofixthis]

Make a few changes to the Optional filter:

• If self.default is callable, return self.default().
• Add an optional parameter to the initializer called is_factory that can override this behavior.

Examples:

>>> f.Optional(default=0).apply(None)
0

>>> f.Optional(default=dict).apply(None)
{}

>>> f.Optional(default=dict, is_factory=False).apply(None)
<class 'dict'>