Book#render patches and sub-section headings

[semmons99]

Two changes to Book#render to accept a single Emitter, and automatically call Emitter.new if an Emitter Class is passed instead of an Emitter instance.

Now I can write

book.render("rbmu-mentoring", Bookie::Emitters::PDF)

Also added the ability to detect sub-section heading /^### / and act accordingly.