Support prefix in environ.Env

[arturponinski]

Sometimes it is desirable to be able to prefix all environment variables. pydantic offers this in this manner:

    class Config:
        env_prefix = 'my_prefix_'  # defaults to no prefix, i.e. ""

Ideally, something like this can be implemented:

import environ

env = environ.Env()
env.prefix = "PREFIX_"

[ecugol]

Just thought about the same thing. Could be very useful for multiple apps running on the same env.

[jayvdb]

This is a feature of environs which is quite handy.
https://github.com/sloria/environs#handling-prefixes

[sergeyklay]

Implemented in #363