Correct Editor.active_chunk

Chunk is inclusive, but selection is not.
Disable rhtml ruby embedding

Since rhtml is a basic mode, and Ruby is now a native mode.
Ruby: lex $<number> globals correctly
Bindings: Specify default for extra_keymaps parameters
Add more documentation

Specifically, start on the "Configuring howl" section, and document the bindings
module.

Also re-organize the doc to hold everything under one banner, and tag the
code example with the type of code they contain.