Create Jinja block for common helpers

[aron.carroll]

Most important is {% snippet %} but {% url_for %} would be great too.

[toby]

{% snippet 'template_name', var=value, var2=value ... %}

{% url_for(controller='foo', action='bar', ...) %}

let me know if you want these changed eg remove need for () in url_for

merged into demo branch they are just available in jinja templates no need to do anything special

Let me know any problems or if you need some others added

[aron.carroll]

Works beautifully. Be great if we could remove the parens around url_for and add an extra one for h.nav_link but I think call it link_for instead.

{% url_for controller='foo', action='bar', ... %}
{% link_for controller='foo', action='bar', class='btn', icon='plus'... %}

[toby]

url_for no longer needs () link_for added but not tested - pretty certain it will just work