feat(linear-progress): Add linear progress indicator

This linear progress directive displays a simple, animated loading bar. There are four display modes: determinate, indeterminate, buffer and query.

Closes #187
Did you test this on Android and iOS? I'm almost sure the animation of `width` and `top` will be really slow there, since those aren't GPU accelerated.

We need to figure out a way to do this animation using `transform`, with translate/scale.

We should screenhero about this sometime.