WIP(Issue#21) - Highlighting man page tokens

Changes in main :

* Added the option of passing arguments into openManPage function and
refactored accordingly.

Changes in token_highlighter :
* Cleaned up code.
* Refactored the higlighter to remove a flicker caused after any changes
in the marked tokens.
* Added event handler to call the callback function on a n event.
* Added click and keyboard listeners to call the event handler.

Closes #21