add configuration file for solarshed.server

[corbinbs]

There are several things that are currently constants within the project.
It would be helpful to allow folks the ability to easily override these things via environment variables and/or a solarshed configuration file. Please introduce support to adjust configuration settings for things like:

• USB port to use for Renogy Rover connection
• time delay in between metric collection from the connected solar charge controller

[corbinbs]

this should help to address some of the items that popped up in #5