Mouse support

[swantzter]

Would be cool to be able to use the mouse to scroll windows line by line (possibly even windows that aren't focused)
Would also be cool to have mouse support in the input to move the cursor

[glguy]

I think the best way to implement this will be to extend the Image' type to handle vertical composition and then decorate that type with annotations that can be found using the mouse coordinates.

Currently I'm converting to VTY's image type for anything bigger than a line.