Use m as export hotkey

The latest org-mode offers export to the ODT word processor format
under the o hotkey (Ctrl-c Ctrl-e o o or Ctrl-c Ctrl-e o O). Adjusting
OPML export to use m as the hotkey avoids clobbering ODT export, and
both options appear in the org-mode export menu.

Signed-off-by: Eric Davis <eric@davising.com>
Update dispatch key combo in README

Closes #6
Accept OPML 1.0 and some other things

Not sure how safe OPML 1.0 is with conversion, but it seems to work with
output from at least one tool that exports it.

This also accepts an encoding declaration on the xml header, and
steps past a comment intervening between the header and the start
of OPML (I'm not familiar with Emacs regexp, the latter tweak may not
really be needed).

Signed-off-by: Eric Davis <eric@davising.com>
Closes #5
Preserve Org markup

This preserves Org-mode inline markup for italics, bold, entities, and
a few other things. Preserving markup avoids the need for
re-conversion when round-tripping of content from Org to OPML and
back, but it might not be the best choice for all tools with which
the OPML is shared. I guess eventually it would be good to have a
set of menu options for the treatment of markup on export.

Closes #7
Force heading if OPML has no structure attribute

Top level is always forced if structure is absent. Otherwise, force
heading if structure has not been encountered yet and we are not in a
list.

Closes #4
Merge branch 'fbennett'

A handful of commits courtesy of @fbennett. See the respective PRs for
more info.