Enable ctrl-shift-enter to reset colors

For issue #78 (closed): Added

Graph.nodeColor(node => (node.color) ?
	 node.color : node_base_color);

to highlight_nodes so that the color get updated

Shifted the search color highlighting (enabled by ctrl+enter) to the else block of evt.shiftKey

if (evt.shiftKey) {
  // Code for reseting colors
  ...
} else {
  // Code for highlighting search nodes
  ...
}

Also changed EventListener to listen for keydown instead of depreciated keypress

Signed-off-by: Adithya Venkateswaran adithyav1511@gmail.com