[WIP] Cron like syntax

There are few things to mention here:

1. File syntax is using a `:` to delimit cron timings from the command definition. This eases parsing, maybe I will remove this later.
2. First run (when there is no mc entry) is still scheduled immediately. So, if you schedule, e.g. to run command at 18:00, then run dcron at 14:00, you command will also run at 14:00.
As we only get a next/prev (expected) run date from croniter, I think it might not be easy to fix without actually parsing schedule ourselves. But any suggestions are welcome :-).
3. Other than 2., I hope it works as expected, but needs to be tested :-).