Add convenience methods for filtering by dates

[kadamwhite]

Possible options:

• .year()
• .month()
• .day()

Does WP support time-frame queries, like before/after?

[brad-decker]

@kadamwhite I'm working on this. Added the convenience methods to the PostsRequest class, and verified that they work. Haven't written any tests or anything.

[kadamwhite]

@brad-decker Awesome! Can you open a PR, so that it will be easier to track and respond to comments?