environment-variables

Is your feature request related to a problem? Please describe.
After scratching my head for an hour or more why a unit test of mine fails (but everything is fine in production) I finally noticed that is configures Dynaconf like this:

settings.configure(
    ENV_FOR_DYNACONF="testing",
    ROOT_PATH_FOR_DYNACONF=os.path.dirname(os.path.realpath(__file__)) + "/../../../config",
    R

I've noticed that when reading a config file with a field that is not available in env, its custom function won't run.

The reason seems to be that for these kind of fields, parseValue won't be called and therefore the corresponding custom function.

Custom functions runs only if the field was provided as an env argument or with

I want to add support for the -v(--variables) operation that currently exists in the gettext version.
more info here

In the README you can find an example related to spf13/Cobra package. We need similar example but for another cli library :)