Added ability to specify minOpacity

Added an optional parameter to options to specify a minOpacity for the heat.
Ah, that's true. You can't currently specify anything though, so I figured it should default to, what I believe the current default is, 0.05. Should I do something like this instead?
```js
var opacity = (this.options.minOpacity >= 0 && this.options.minOpacity <= 1) ? this.options.minOpacity : 0.05;
this._heat.data(data).draw(opacity);
```