Snap multiple relative points; 'startCoords' offset

[taye]

interact(target).draggable({
  snap: {
    targets: [ {x: 300, y: 300} ],
    relativePoints: [
      { x: 0, y: 0 },  // snap relative to the top left of the element
      { x: 1, y: 1 },  // and also to the bottom right
    ],  

    // offset the snap target coordinates
    // can be an object with x/y or 'startCoords'
    offset: { x: 50, y: 50 }
  }
});

relativePoints

The snap.relativePoints array succeeds the snap.elementOriign object. There are effectively ( targets.length * max( relativePoints.length, 1 ) ) snap targets when snap calculations are done. Snap functions are called multiple times with the coordinates of each relativePoint. If no relativePoints array is specified or the array is empty then snapping is relative to the pointer coordinates (default).

Backwards compatible – elementOrigin still works with the old snapping interface.

interact(target).snap({
  mode: 'anchor',
  anchors: [ {x: 300, y: 300} ],
  elementOrigin: { x: 0, y: 0 }    // only one elementOrigin is possible
});

offset

The x and y values of the snap offset are added to the target coordinates. If the offset is 'startCoords' then the pageX/Y of the down event that started the action are added to the target coordinates. See #135 for more info.

Closes #132
Closes #135