Add more helpers

[siiptuo]

Currently this has only url_for function. The old Slim-Views has 4 different helpers. A simple solution would be to add Slim\App as a global. It would look like this:

{% if slim.request.isGet %}
    <a href="{{ slim.request.uri.basepath ~ slim.router.urlFor('home') }}">link</a>
{% endif %}

It's not as pretty as few selected helpers but the API is would same as in PHP code. Unfortunately it would also make it possible to include application logic into templates which may have unwanted side effects.

[akrabat]

I'd rather see a PR for specific additional helpers.

[silentworks]

These should probably be split back out as the original was, so we have a separate class for the helpers in the repo.

[akrabat]

See #22