viewmd: features & mathjax.

Features can be activated from the first line of the document, v.g.

<!-- viewmd: mathjax hilite -->.

Moreover, the default FEATURES can be overridden in the configuration file. The
"default default" features include just "hilite" because it's expected that many
documents include code and because it can be enabled even when the document
doesn't really need it without significant loss of performance (unlike mathjax,
which adds a lot of unnecessary overhead).
vim: markdown <...> style urls, firefox new window.

<Leader>o map now supports both <...> and [](...) style urls.

OpenUrl(url) opens url in a new firefox window, not in a new tab (same window).