Control-C to kill decoder.

[galenhuntington]

Closes #147, and more about #106.

• Control-C now kills the decoder if it is running.
• If it is not, the exit modal is popped up. Thus ^C^C offers to exit.
• As before, hitting ^C repeatedly eventually quits. In normal flow, ^C^C^C exits.
• Code is tidied a bit with unneeded special-casing removed.
• I had to add a transformers dep explicitly (it was already a transitive dep).