Add --drafts flag

I really liked the [_drafts folder](https://groups.google.com/d/msg/jekyll-rb/uUCuQwNMWaQ/Gxt36HjegZAJ) idea suggested by @mojombo a while ago:

> It might be nice to have a --drafts switch that just picks up
and publishes stuff in a _drafts directory if it exists.

Related: #513