Slightly clean template.rb
Leverage the "addressable" gem to clean up all our URL templating by replacing them with an RFC 6570 URI templating implementation.

URI parameter extraction and substitution is now totally handled by addressable. We can now support RFC 6570 templates in addition to Sinatra-style colon-var templates for our templated URIs - in fact, RFC 6570 templates are now the default. Also, permalinks can now include data from any page frontmatter key simply by referencing it as a variable substitution.