Added `itemToMove` option allowing 'menu' || 'panel' || 'both'

[AaronLayton]

Reference issue #140

Sorry it took so long to get this pull request in but here is the modifications I made to allow the user to drag out the menu.

Usage

var slideout = new Slideout({
  'panel': doc.getElementById('panel'),
  'menu': doc.getElementById('menu'),
  'itemToMove': "menu" // menu || panel || both
});

The code was very rushed in as I need it for a live project with a deadline so I am sure this can be optimized a lot more. For example I essentially had to re-initialise all the values in _recalculateAll() - not great but it got the job done for what I needed.

[AaronLayton]

Any news on this PR?