Support copy and paste from output

[dhleong]

The reason we disabled mouse support by default was probably because it breaks the terminal's built-in copy and paste. We can now disable mouse support on the fly, but we should probably consider how to do a visual selection mode in an output buffer to support copying more easily.

The most robust solution is a normal mode that navigates in the output buffer (we probably don't want to get into editing the output buffer) and can do selections with v. A less involved solution might be to handle mouse events to create selections, so we don't have to deal with focusing on and navigating in the output buffer.

[dhleong]

We could maybe use the <c-w>N mapping from vim's terminal mode to switch to "output normal mode" and jumps into the output window