Make WTP more modular

[eniraa]

As it stands currently, WTP seems to have code that is rather specific to StackOverflow. For instance, there is code that is specifically for navigating questions in display.py: https://github.com/what-the-python/wtpython/blob/022785fa710f3bf69becd0b2a429e4926f37ba2d/wtpython/display.py#L147-L161

Perhaps this code would be better located in some file which is specific to SO like stackoverflow.py.

I think it might be a good idea to have the application be more modular (i.e. allow someone to replace SO with RTD without too much work). This will also help in implementing features like #106.

[briancohan]

This is one of my main goals. I want to make it so the display class manages as little as possible and has the stackoverflow object track which question it's displaying as well as the displays for the sidebar, main window, and even the --no-display option. Then I hope to incorporate a search through the Reddit API and other sources of information.