Directive to highlight unused tokens through depTree plugin

Closes #207 

Previously we kept track of a count of unused tokens right through the `depTree` service.
This is now driven out to an own directive called `unusedTokenHighlighter` that ships with the `depTree` plugin.

This directive displays the same text as before, but it is now clickable: A click will add styling to tokens (has a default value but can be customized through passing the attribute `unused-token-style="{ key: value }"`)

A tooltip notifies the user that something will happen when he clicks on this element.

In the process of coding this directive better style handling in general was added (check the new functions in the `state` object, how `morph` uses them and new watch behaviour of the `token` directive).