More options for scheduling/fetch interval

[Grizzelbee]

I like to do two different things with this adapter:

• Monitor the current power consumption constantly
• fetch once every day the current total power consumption value and store it in the database at the best every day at the same time

Monitoring the current power consumption works currently perfect so this is no issue.

Currently I have two instances for that:

• instance 0: Monitoring with an intervall of 25 seconds, storing the current power consumtion
• instance 1: daily fetch with an interval of 24 hours, storing the total power consumtion

Using two instances is fine for me, but I like to have the opportunity to define a fixed time for the data pull, like:

• every day at 23:59
• every last day of the month at xx:xx
• every first day of the month at xx:xx

to summorize: simple cron scheduling.