Add schedule feature and tests

[schmorrison]

Thanks for maintaining this package!

Simple scheduling mechanism to periodically start a Collector's scraping routine. The entire schedule group can be cancelled via StopSchedules or cancelling the Collector.Context. When a stop is executed, all currently running schedules will finish naturally (not at once). Currently, the Collector's registered callbacks are not aware of the scheduleContext cancellation via StopSchedules.

I'm sure you will like me to tweak the implementation here.

[WGH-]

This uses public Colly API only, you could do it from your own package just fine, and I'm not yet convinced this is frequent enough use case to be integrated into the main code base.