Feature/limitSliders Labels

[billyz313]

This is a request for the limitSliders to have some type of label while dragging that displays the current date they are on to give the user context to where they are setting the date range. Currently I am adding a mouseup and touchend listener to .knob.lower and .knob.upper and manually updating a date range display. This is not the best approach because it only updates the date when the user drops the knob, it would be best if we could update it as the user slides it. I had tried adding a mousedown/mousemove event and updating, but map.timeDimension._lowerLimit and map.timeDimension._upperLimit only update on the final drop. Is there some property i can access to find the value the knob is over while it is being dragged so i can update in real time?