Fix dispose to be similar to init (adding event)

[nicolasigot]

Hello,

I have a use case in which I need to dispose metismenu on an element and recreate it later.
The issue is that the dispose does not off the click event on the same elements and same event as the init function.

This leads in error in console

The fix is to update the dispose to select the same elements and use the same event.

Thanks,
Nicolas