Previously Nuzzle created a `:nuzzle/render-content` function for each
page based on the `:nuzzle/content` value. Now `:nuzzle/content` is
removed and users must specify their own `:nuzzle/render-content`
function for every page. If they don't include a `render-content`
function, a function will be added that constantly returns `nil`. This
allows the user to have complete control and understanding of how each
page's content gets made.
The `nuzzle.content` namespace has been reformatted and will probably be
merged with `nuzzle.hiccup` soon. `nuzzle.content/md->hiccup` is an
important function that can be used to include markdown files for page
content.
Fixes #134