Allow setting a label to describe the current progress stage

[felipesanches]

I'd suggest adding to the API a function called setLabel that provides a string describing the current progress stage. This optional label should be rendered inside the progress bar area. The label should be undefined by default resulting in the current ngProgressbar behaviour.

[victorb]

Hm, I'm not sure about this but worth discussing. I feel like it would add more to the progress bar than what a progress bar really is, and it's probably more suited to provide in your own application, rather in this library.