Allow users to declare safe env files in .dotenv-stripout-ignore, whose values won't be stripped

[harrisonpim]

This PR closes #5, following the discussion in #1

It adds the option for users to include .dotenv-stripout-ignore files in their repos, which lists a set of files which should not be stripped. For example:

my_project
├── src
│   └── ...
├── .dotenv-stripout-ignore
├── safe.env
└── unsafe.env

the .dotenv-stripout-ignore file might look like

safe.env

unsafe.env might contain

# here are some comments
# i can add comments however i choose
THIS_VARIABLE_SHOULD_BE_SECRET=
SO_SHOULD_THIS_ONE=

and safe.env might contain

THIS_VARIABLE_IS=okay_to_share
AND_THIS_ONE_IS=too
# also able to add comments in this file, in whatever format I want!

The variables stored in unstripped files should represent public config, ie non-secret values which are safe to share publicly. Adding this feature matches the recommendations in https://12factor.net/config nicely.

In this implementation, the path discovery for patterns listed in the .dotenv-stripout-ignore file(s) approximately matches the behaviour of a .gitignore file. The major changes are made in dotenv_stripout/stripout.py

Note: Before the next release of the package, this PR should be followed by a more complete update to the project's documentation to help users understand the feature.