Use axisLabelColour (if set) for html labels, or omit inline css color

As per [issue #22](https://github.com/markrcote/flot-axislabels/issues/22):
>HTML labels are currently hardcoded to use `this.opts.color`, which is used by flot for colouring gridlines. Since this is set as inline css, it's impossible to override with a stylesheet (using eg `.axisLabels {color: black;}`) and effectively means that the axis labels have to be the same colour as the gridlines.

>This should perhaps use `this.opts.axisLabelColour` if it's been set, and otherwise omit the inline `color` declaration so that some global style can be applied.

>Specifically the line
>```javascript
>'color: ' + this.opts.color + '; ' + 
>```
>should be replaced with something like
>```javascript
>(this.opts.axisLabelColour ? 'color: ' + this.opts.axisLabelColour + '; ' : '') +
>```