Makes a Twig filter called apply_search available, which can be used in
templates to
filter the pages array on demand. Because of that, we can stop mutating
the pages array, which other extensions and themes may rely on.
Addresses #6, which highlights some confusion about the example search form.
This updates it to use the `link` filter used in Pico templates to render URLs
appropriate for the value of the `rewrite_url` config variable.
