Rearrange repo layout so that it's suitable to use with git-submodule

[axgfn]

Since nobody wants to do the work in #18, it would be nice if people could easily include the pandoc plugin as a submodule in their wiki repo (with some config changes in *.setup of course).

See here for an example ikiwiki site that bundles ikistrap as a submodule.

This would involve moving page.tmpl to a templates dir, pandoc.pm to lib/IkiWiki/Plugin, and testing & updating the installation instructions. I'm not sure where mathml.js would go.