Highlights design

Continuation of #257, just that it is a pull request against development branch. This pull request is addressing #256. Instead of each highlight being one big rectangle area, find smaller reasonable sections and draw border around only them.
Put comment above this line, especially with such a long line. If it's something that yet needs to be done, split it to a ```# TODO:``` comment.

Store ```15``` into a constant with some meaningful name like ```MAXIMUM_MERGE_GAP = 15``` to the top of the document.