Investigate why jpterm hangs when used in command substitution

jpterm does not work and will just hang when used with `$(...)`. For example, this works:

```
echo '"foo"' | jpterm
```

But this will not:

```
echo $(echo '"foo"' | jpterm)
```

I'll need to investigate if this is just a limitation in urwid, or if there's something in jpterm.py I can do to enable this scenario. This would enabled things like:

```
aws s3api delete-objects --bucket foo --delete "$(aws s3api list-object-versions --bucket foo | jpterm)"
```

So you could get the JSON structure right by messing around in jpterm, and then that result would then be used as the JSON input structure for the `--delete` arg.

Traceback (which occurs when I try to Ctrl-C out):

```
$ echo $(echo '"foo"' | jpterm)
Traceback (most recent call last):
File "/usr/local/bin/jpterm", line 6, in
jpterm.main()
File "/usr/local/lib/python2.7/site-packages/jpterm.py", line 194, in main
display.main(screen=screen)
File "/usr/local/lib/python2.7/site-packages/jpterm.py", line 137, in main
self.loop.run()
File "/usr/local/lib/python2.7/site-packages/urwid/main_loop.py", line 278, in run
self.screen.run_wrapper(self._run)
File "/usr/local/lib/python2.7/site-packages/urwid/raw_display.py", line 272, in run_wrapper
return fn()
File "/usr/local/lib/python2.7/site-packages/urwid/main_loop.py", line 343, in _run
self.event_loop.run()
File "/usr/local/lib/python2.7/site-packages/urwid/main_loop.py", line 673, in run
self._loop()
File "/usr/local/lib/python2.7/site-packages/urwid/main_loop.py", line 697, in _loop
ready, w, err = select.select(fds, [], fds)
KeyboardInterrupt
```