Add Selmer integration for content preprocessing

[stelcodes]

Currently there is no way to template HTML and Markdown content. Adding an integration with Selmer could be amazing.

Ideas:

• Use the page entry map as the Selmer context so HTML/Markdown can inline values straight from the Nuzzle config. Or have something like:

{:page <page-entry-map> :config <whole-config>}

• Allow users to define their own filters in a config option map

:nuzzle/selmer
{:filter-fns {:embigenate my.ns.embigenate}}