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
`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>