Allow custom classes to be passed to nav.tabs() helper
Allow custom classes to be passed to nav.tabs() helper

Usage:

```
{%
    set nav = [
        {
          "id"    : "foo",
          "label" : "bar",
          "src"   : "baz",
          "class" : "one two three"
        }
    ]
%}
{{ tabs.list(nav) }}
```

Closes #567

jadu/pulsar,jadu/pulsar,jadu/pulsar