Generalize ActionBuffer inserts to accept "styled objects"

Previously it only accepted Chunk:s, but now that's generalized
to accept "styled objects". These are simply tables containing
`.text.` and `.styles` fields.
Renamed command input -> external command input
Add StyledText class

For holding styled text. Tries to be as string-like as possible.
Add first markup parser..

Howl markup - A simple HTML like markup for creating styled objects.
Document howl.ui.ActionBuffer
spec2html: Less wordy
Readline: Support StyledText for prompt and text

The text part being less tested, so we'll see once actual
usages of this turns up.
Use a fancy styled prompt for the command input