Allow users to exclude index args when creating requirements.txt

Is your feature request related to a problem? Please describe.

Currently the output of pipenv lock -r includes index arguments (e.g. -i https://somehost/simple) unconditionally (see https://github.com/pypa/pipenv/blob/d69198c0fb13a0ff92a6d105f2eb07f23ceb70b5/pipenv/core.py#L849). If you are using different repositories for your local environment and production (e.g. if you are using devpi locally), you need to manually delete those lines before deploying to production.

Describe the solution you’d like

I would like it if the inclusion of index arguments in requirements.txt was configurable, through a command-line flag or a setting in the Pipfile (or both)

Describe alternatives you’ve considered

An alternative would be to exclude the arguments when --pypi-mirror is set to an empty string when executing pipenv lock

[kaptajnen]

The new command has not changed this behavior so it is still required to run pipenv requirements | sed '/^-/d' to get the desired output.

[matteius]

@kaptajnen would a --no-index option to the requirements command that excluded any -i <index> lines be a suitable solution?