Catch SIGTERM signal to gracefully terminate

[pataquets]

While dockerizing the app, I've found that it ignores SIGTERM signals from both CTRL+C and docker stop commands (it just gets killed after 10 seconds in the later case).
Trapping SIGTERM signal to terminate gracefully would prevent this annoyance.
Thank you for this useful app.

(Same issue as hgoebl/nerdshow#2, if it's of any help)