Implement indeterminate circular progress

[nero2009]

Please how can I implement an indeterminate circular progress.

[apfritts]

There are a few other issues that also request this feature: #29, #253, and #55.

#29 has some code in it for hacking this.

As a formal feature request, I think it would be awesome to add a loading: boolean property that when true automatically animates the progress bar circling in the visible area.