Add graph.attachBefore

The idea for calling a function before the method it is bound to is to provide pre-processors, for instance:
```js
var colorPalette = { Person: '#C3CBE1', Place: '#9BDEBD' };
graph.attach('addNode', 'applyNodeColorPalette', function(n) {
  n.color = colorPalette[n.category];
}, true);

var myGraph = new graph();
myGraph.addNode({ id: 'n0', category: 'Person' });
console.log(myGraph.nodes('n0').color); // outputs '#C3CBE1'
```