Add preliminary support for Opera Mobile

We do this the lazy way, using use_other_browser_cmd("opera %s").  The
Opera Labs post announcing Opera Mobile for Maemo
(http://labs.opera.com/news/2010/05/11/) says that "an extra empty tab
will get opened every time you open a link from an external
application", so I suspect we may need a different method of opening
links in an already open browser.