add onDropText event to support text dropping

[mathiasvr]

Hi, I was taking a look at the webtorrent/webtorrent-desktop#284 issue, and ended up adding a onDropText event in the drag-drop module. It is used like the other extra events:

dragDrop('#dropTarget', {
  onDrop: function (files, pos) { console.log('Dropped files', files) },
  onDropText: function (text, pos) { console.log('Dropped text', text) }
})

I don't know if this belongs here, since the module is based around file dropping, but this PR keeps the module backward compatible and only changes the modules behavior if the onDropText event is registered (adds drag class for strings etc.).

However we can probably get this to work directly in WebTorrent Desktop if you prefer.

[feross]

Thanks for the PR @mathiasvr, but I saw #25 first and already merged that. I ended up adding an onDropText and making many of the same changes that you made :)

[mathiasvr]

@feross No problem, nice work. 👍