tpl: Add a querify function to generate query strings inside templates

tpl: Add a querify function to generate query strings inside templates

The query function will take a set of parameters specified like a dict and return a url.Values object which can be .Encode'd into a query string.

Example:

<a href="http://www.google.com?{{ (querify "q" "test" "page" 3).Encode | safeHTML }}">Search</a>

Returns:

<a href="http://www.google.com?page=3&q=test">Search</a>

Closes #2257