method to save as _draft, and _draft to _post

This adds functionality that takes advantage of Jekyll's _drafts feature.

Users have the option of saving posts and edits as a draft. This creates or updates a file of the same name in the _drafts folder. Building the site with the `--drafts` flag will render these and give priority over _posts with the same name.

_drafts can then be converted back to _posts, again either creating or updating a file of the same name in _posts. The _drafts version is then deleted.

/cc @tristen @mikemorris @dhcole