Added mouse support for tig

Added the following features:
- Clicking on a line in a view will select that line.
- Clicking on a line in the main line that has already been selected will "enter" that line (e.g. show the diff view if clicking a commit in the main view)
- Clicking on an unselected line in the main view while the secondary view is open will "enter" that line
- Clicking a view will select that view as well (i.e. change the current_view variable)
- Scrolling the wheel up/down will scroll the view underneath the cursor up/down and select that view
- Mouse support and the scroll distance can be specified via options

I've got mouse enabled by default, but I wasn't sure if this was a good idea, since people might encounter issues re: text selection. The default can be changed by setting opt_mouse to FALSE.
Fixes #157.