Consider removing the .onLoad functionality

[dshemetov]

The function in #13 is especially useful when you want to load variables, but not overwrite your env vars. This is moot due to the .onLoad function in this package, which eagerly loads the env vars. FWIW, the python-dotenv package also separates package import and the env var load.

I understand if the migration/backwards incompatibility is not worth it for the maintainers, but just want to open this idea up for your consideration at some point in the future.