Add ProgressBar option to Crouton

Occasionally I use a Crouton to let a user know when an operating is happening in the background, in these cases I find it helpful to include a small visual indicator that "stuff is happening" in the form of an indeterminate ProgressBar. This path adds the *option* to include a ProgressBar, using the same patterns as the other optional elements. 

NOTE: I also added an "Info with Progress" bar item to the sample, so this can be quickly viewed, though that may be overkill.