Add ability to read config variables from env

[altexdim]

Add ability to read config variables from env

Usage example:

DONKEYCAR_CFG_MAX_LOOPS=2000 python manage.py drive

this command will override config variable MAX_LOOPS and set it to 2000 this is the equivalent of having MAX_LOOPS=2000 in the myconfig.py

[DocGarbanzo]

@altexdim - do you want to address the comments so we can merge this?

[altexdim]

Yes, absolutely, thank you for the review, I'll fix it as I find some time

[Ezward]

@DocGarbanzo if we like this feature (I like it), we should change that to f-string ourselves or merge as is.