Support Drafts with "showDrafts" in options and "isDraft" in a post's properties

This is mostly a follow-up to this pull request https://github.com/jsantell/poet/pull/8
The code is based on it except I prefer a post property as opposed to a tag.

So, by default, showDrafts is set to true unless NODE_ENV is production. When showDrafts is true, any posts with isDraft == true are included in any getPosts() (or related) calls.

This pull request includes test cases as well.

Disclaimer: I just started working with node.js so be gentle ;-)