Add data-step feature

[breakbuidl]

Its a useful feature because different projects use different scale to store ratings. With the help of data-step along with data-min and data-max, people can create their own scale very easily.
Please let me know what you think.

[javiertoledo]

I feel like you could easily map the integer sequential values to your application values in the server side, so not having this feature shouldn't be a stopper, I'd prefer not to merge this to keep the rating input code clean, but I really appreciate your PR and the idea. If anyone else votes for this to be merged, I'd reconsider.