Added helper splitOnNumbers to sketch and tests for it

Leslie,
I've added a helper method to split the string on numbers, and I've used it for the textBits, however I'm not sure about that attribute name or value to subscript numbers, so you should check that.

I also added tests for that helper method including one test where your real code showed me unexpected behavior, which I wrote the failed test for and then fixed.

Also this is likely to cause a conflict w/ the bits in widget-sketch we were playing with, but I think this is what you want.