Add data-labels attribute to turn knob into a multiple choice switch

[wakandan]

add data-labels attribute which accepts a comma delim string as a list of labels to be displayed on the dial. Good in the use case of making the knob into a multi-valued switch, which could be used as an option button in form or similar. Searched for a while but haven't seen this feature in any other libraries

P/S: there seems to be a lot of changed but actually it's caused by prettify plugin from sublime.

[eskimoblood]

First, thanks for the pull request. But please when you make the next pull request don't use the code formatter. It makes it really hard to figure out what was actual changed.

[wakandan]

My bad on the auto formatting, seems like a bad habit. I only tried out if it works initially. Will clean up the code at those places and create another pr. Thanks.