--env-override should apply to all config

[jefflester]

The --env-override flag currently applies to the .env file in the root of the library. A user should be able to override any variable in this .env file, and they should be able to override any variable set in their minipresto.cfg file. Override checks need to be updated in core.py:

• Make a dictionary of all variables from the environment file and Minipresto config file
• Run each of those variables through the override check if an override is present
• Return the updated dictionary

At a higher level, this raises the question of how to best manage config. Does it make sense to have it all live in one file? Probably. Perhaps everything should live in the minipresto.cfg file, the library's root .env file should be deleted.