title: How to display a tooltip on the navigation controls in mapbox-gl-js?
body: <p>I am building a mapbox-gl-js application for very inexperienced users and looking to provide as much guidance as possible.  The standard icon buttons for zoom in / out/ gelocate etc are great but as an extra bit of help I would like to display a tooltip on mouseover with a textual description of what the button does "Zoom In", "Zoom Out", "Zoom to current location" etc.  </p>
  
  <p>I have checked the API docs and looked at the source code in github but cannot see an option to provide this customisation.  If anyone else has successfully done this or have some suggestions on how this could be achieved without forking mapbox I would be very grateful!</p>