Check if a cron job is running before starting

Hi there,

Don't know if this is something you would be interested in integrating into the main branch, but here is a sample implementation that works - checks if a cron job is already running via fcntl.lock and then either runs the function or not.

Does not require any additional python libraries or anything.

Would obviously ideally be wrapped around a setting that controls if this check should be done or not (wouldn't want to change the behaviour for existing users without them explicitly asking for it I guess).

Basically posting to see what you think of this.

Cheers