Add support for example option passed through env variable

[jraoult]

There is currently no way to set the example option using an env variable. As rightly pointed out here in this issue, DOTENV_CONFIG_PATH is supported thanks to dotenv, but since it is not aware of dotenv-safe's example option this one is ignored.

I added the support for DOTENV_CONFIG_EXAMPLE in dotenv-safe/config.js. I got inspiration from what is done in dotenv/config.js being more restrictive than my initial patch and only explicitly supporting DOTENV_CONFIG_EXAMPLE

config.js is not covered by the current test suite so I'm unsure how to proceed. The manual way of testing is typically the following command:

DOTENV_CONFIG_PATH=src/.deploy.env DOTENV_CONFIG_EXAMPLE=src/.deploy.env.example node --require dotenv/config --require dotenv-safe/config src/myApp.js

It would fail before with Error: ENOENT: no such file or directory, open '.env.example' but is now supported.

closes #126 (comment)

[rolodato]

Thanks @jraoult! No worries about the tests for now. I'll release a new version soon.

[rolodato]

9.1.0 is now released. Also added support for setting DOTENV_CONFIG_ALLOW_EMPTY_VALUES, for completeness. Thanks again!