Show hg branches in the prompt

Actually, show branch+bookmark, except if there is a bookmark on the
default branch. There are also extra templates for explicitly showing
just the bookmark or just the branch.

Also added a corresponding explanation on the docs.
Add an hg style to prompt templates, reusing existing git style
Add a quiet option to suppress startup message during playback
Add an option to echo coments.

This is useful for annotating your session. During playback, the same
annotations in your session can be optionally displayed for the
benefit of the audience.
Add a --comment-echo CLI param
PEP 8 style fixes