Make scheduling/sleeping more granularly and generic

[kaerbr]

First of all:
I really like this project. Thanks for creating and sharing! (:

In my opinion I think the sleeping/scheduling capabilities of the code could be improved. To even prolong the battery life it would be beneficial if I could individually define the sleeping and not sleeping hours of each day. If I'm not home the display doesn't need to be refreshed.

I even coded a algorithm that I will add as PR. If you think this fits in the project I would be happy.

I guess this is linked to #157