Mouse event on hover

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

In complex graphs, the interactive hover can be very useful to highlight the connections the user intends to follow and this feature is apt for such observations. On hovering over a node or edge, the edges which are a part of that network get highlighted. There is also an option where the label can be interchanged when the cursor is on the node and when it is not on the node. ccNetViz also keeps a track on the history of hovers where the nodes and edges hovered over are stored in order in separate arrays. This feature can be utilized for more customizations like changing the node label for each hover, changing the color after certain number of hovers etc