Add minimal asciidoc syntax

Add minimal asciidoc syntax

Though this protocol description is mainly to be viewed as textfile, a few
minor changes make it parsable for asciidoc to spit out reasonably
nicely-formatted html code.

Changes include:
- underline section headers with the matching lines
- add linebreaks before lists to parse them as lists
- change indentation level for normal text to be left-marging aligned and
  for <pre> text to be indented
- comment out section dividers

It's possible to run asciidoc XI2proto.txt and get some nice html output
now.

Reviewed-by: Gaetan Nadon <memsize@videotron.ca>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Chase Douglas <chase.douglas@canonical.com>