Allow running one-off for debugging purposes

Currently stapled is engineered specifically to run as a daemon perpetually in the background but for testing that is rather inconvenient, especially in CI. Allowing it to run one-off also allows other uses such as prepping a staple for servers that can serve stapled from a file and can also fetch staples but don't do so until a request has been made to the server (nginx and apache, I'm talking to you..).