Added option to filter mousedown

[EduardoRG]

I wanted to disable panning unless altKey is pressed. The proposal allows to disable totally the mousedown panning or filtering by key. I think it's related to #72, #90 and maybe #133. Added beforeMouseDown() option to docs.

E.g.

panzoom(element, {
  beforeMouseDown: function(e) {
    // allow mouse-down panning only if altKey is down. Otherwise - ignore
    var shouldIgnore = !e.altKey;
    return shouldIgnore;
  }
});

[992forever]

Hello, I also need to disable panning in my project. Why didn't I find the "beforeMouseDown" property method in version 8.6.1? Looking forward for your response, thank you

[anvaka]

Thank you, published as 8.7.0