Add CronCommand - a command that can only be run from the command-line.

This represents all we'll do for #1477

Why you'd want this:
* To interact with chronograph easily, but to force the command to be run within the server process.
or, in other words:
* To guarantee that updates that could invalidate caches only happen within the server procedure, so that necessary global variables and caches can be cleared.
or at least,
* To guarantee that someone smart enough to know how to run the command from the command-line is also smart enough to know that they'll need to restart the server and manually clear caches.

Changes:
* Add the CronCommand abstract command class
* Have `videodownload`, `videoscan`, and `languagedownload` implement that abstract class.