feat(progressbar): Adding units and custom texts to bars

I've added the possibility to specify units and print them inside the bar being the default the % symbol. To do so, it's only needed to add the attribute with-units as a scope variable or string i.e. `with-units="myUnit"` or `with-units="'ºF'"`.

It's also possible to specify any custom text with the object format in the 'text' property:
```javascript
$scope.myObj = {
  value: 10,
  text: 'Ten'
}
```