feat(tabs): add support for setting basic bootstrap tab-types and tab-styles via directive attributes

This change improves the flexibility of `bs-tabs` without affecting default behavior, by allowing the directive to be configured (via directive attributes) to render as any allowable combination (see below) of bootstrap's `nav` components. The `bs-tabs` directive now supports using `nav-pills` instead of `nav-tabs` by setting the directive attribute `data-use-pills` to a truthy value.  Additionally, you may declare that either tabs or pills be `nav-justified` when `data-justified` evaluates to "truthy".

Attributes are evaluated against the appropriate scope, so they don't need to be hard-coded (if, for some reason, this result needs to be produced conditionally). 

Finally, if both `data-use-pills` AND `data-stacked` evaluate to "truthy", then pills will be stacked instead of inline (doesn't work for tabs, as that wouldn't make much sense... Technically, it is possible to stack tabs, but I haven't allowed it).

I think this is a useful change in that it makes the basic bootstrap "nav-tab" and "nav-pills" configurations accessible without requiring a custom template, though it will provide the same functionality to a custom template, if necessary.