Allow specifying individual check intervals for each item

[n-st]

This issue is basically a clarification of #148:
Like @strauhmanis, I want to check different sites/commands at different intervals.

So far I've resorted to having urlwatch --urls=urlwatch.5mins.yml and urlwatch --urls=urlwatch.hourly.yml in my crontab, but this approach doesn't scale well (I now want to add daily checks for some sites).

We already have a database as the back-end (though I don't know the specific layout), so couldn't we just keep a "run next check at or after xx:yy" field in there and run urlwatch from cron every five minutes?