Add support for environment variables at pyproject.toml

It would be handy to have support for using values from environment vars on pyproject.toml The use case I have is that I need to use different repository urls depending on my environment. Something like:

[[tool.poetry.source]]                                                          
name = "private"                                                          
url = "${PRIVATE_REPO_URL}"

I’m still new to poetry, but I could help implementing the feature with some guidance.

[SpicySyntax]

What should we do in the case that the url for the private repo contains private username and password? I.E.

[[tool.poetry.source]]                                                          
name = "private"                                                          
url = "https://${ARTIFACTORY_USER}:${ARTIFACTORY_APIKEY}@artifactory.corperation.com/artifactory/api/pypi/pip-local/simple"

[Peque]

Pipenv implements this feature. As they mention, it is “quite useful if you need to authenticate to a private PyPI”. That is my case too.

@sdispater Being ranked as the 11th most popular issue in this repository (out of 400+), would you reconsider accepting pull-requests for this? 😇