Introducing "rangeSelectorColor" option

[haschu]

With the rangeSelectorAlpha-option, we can define the transparency level of the range-selector, but there is no way to set the actual color.

It would be nice to have a rangeSelectorColor-option as an alternative where one can set the whole color-(RGBA)-string.

Currently we have the following implementation:

ctx.fillStyle = 'rgba(240, 240, 240, ' + this.getOption_('rangeSelectorAlpha').toString() + ')';

I would suggest something like:

var fillStyle = this.getOption_('rangeSelectorColor').toString();

// If not defined, stick with the old way to be backwards compatible
if (!fillStyle) {
  fillStyle = 'rgba(240, 240, 240, ' + this.getOption_('rangeSelectorAlpha').toString() + ')';
}

ctx.fillStyle = fillStyle;

I could come up with a pull request, if you guys are okay with that solution :-)