support step offsets

I have used rules like these in my system crontabs for a while.

On www1:
7-59/20 * * * *

On www2:
17-59/20 * * * *

This handy declaration allows running cron somewhere every 10 minutes, or every 20 minutes if one web server is down. We're evaluating moving to Elysia cron and found that this rule doesn't work:

7-59/10 * * * *

The expected run times are then 7, 17, 27, 37, 47 and 57 minutes past the hour. We use this to balance loads for many things that run every 10 minutes by adjusting the offsets. Elyisa cron currently ignores the part before the slash and reads this as if it were:

*/10 * * * *