Add linear/exponential mode to zoomslider

This pull request introduces the config option `mode` for zoomslider controls. 

Previously a dragged amount would have been translated to a resultion in a linear way only, so that a simple zoom-in in the map would make the thumb jump a rather big amount. When the control is configured with `{ mode: ol.control.ZoomSliderMode.EXPONENTIAL }`, this will not happen anymore.

The control still defaults to the linear behaviour, but I think that is debatable.

The (mis-)behaviour has been mentioned in #380.