Insta-pipe

Quickly iterate on complex shell commands.

Edit commands in the top pane, and each time you press enter the first n lines will appear in the preview pane.

TODO

Avoid blocking until the command completes, by reading the Popen pipe line by line. (Will require a working flush).
Do some basic caching to avoid recomputing repeated results.
If the user exits with ctrl-D, insert the final command into their shell session somehow. (Paste buffer?) For now, we just print it.

Known Issues

all backends
- terminal color escape codes aren't supported in the output frame.
curses backend
- If you press enter with an empty input pane, instapipe exits.
- Instant mode isn't supported.