Custom icons and styles

Greatly appreciate the work done in bootstrap-rating-input, but my project needed to use a different icon and display style.

These additions enable just that by adding three optional data attributes
data-base - base class, always used regardless of state (used for setting the icons)
data-classOn & data-classOff - classes which control how the base icons are displayed (e.g. colors for on/off state)

I haven't exhaustively tested this but it works for my use cases.

_Daniel