Add option to run `/podcasts/check` on startup

[reibitto]

This should be a config option. Something like checkForUpdatesOnStartup.

This kind of overlaps with #8, but only slightly. #8 is more for when you're running the Podpodge server over a long period of time. This checkForUpdatesOnStartup option is more useful for when you simply want to run the server every once in a while to download some new episodes, and then kill the server when you're done.