Fixes #13347 (closed).
Basically for some stupid reason I scoped striped progress bars to an immediate parent class. That's just silly, and this gets around that by adding a second selector to target just .progress-bar-striped. Essentially, just add it to an existing .progress-bar and you get stripes overlaid.
Probably need to add some docs for this and perhaps deprecate the old shit.
/cc @tristanlins