Mouse events on move

Live example: https://vchilkuri.github.io/examples/interactivity_move.html

This feature adds more interactivity to the graphs where the user can click on the nodes and drag them to the required point. This feature also helps in grouping nodes and networks into subsets and thereby useful for analysis.

The graph.reflow option which comes into action once all the mouse events occur and then the node movement is completed. It makes all the dynamic changes static to optimize the performance of the graph and make it ready for next action to be performed.