Fixed a typo
Documenting metadata options
Switched (rather painlessly) to using swig

Swig is very much like Jinja and includes a lot of the powerful features I really liked
in Jinja and was missing in Handlebars.  This is already paying dividencds because I can
define macros right in my slides.  But it also opens to door for extending from templates
in a really easy way too.

+1
Changed the markup to allow a third kind of fragment
Added support for extending from templates