Allow environment variables in schema definition

[nurikk]

Hi! This PR introduces environment variable substitution for schema configuration

Now you can craft config file like this:

    tables:
     - auth_user:
        search: id  = '${USER_ID_TO_BE_DELETED}'
        fields:
         - first_name:
            provider:
              name: clear

and then run
USER_ID_TO_BE_DELETED=145 anonymizer

[hkage]

Hi, thanks again for another contribution 👍 I will take a look at it as soon as possible. The feature looks promising and useful.

[hkage]

Looks good to me 👍 Thanks for the pull request!