Add optional pipenv install stage

[Tobotimus]

Hi!

I have a patch here for #3181. I went into some detail of my thought process on that issue, so I will do a quick summary of what I've done in this patch:

• A new pipenv section in the top level of the YAML configuration, with keys enabled (bool) and options (list of strings). Here is an example:

  pipenv:
      enabled: True
      options:
      - "--dev"
      - "--ignore-pipfile"

• If pipenv.enabled is True, pipenv==2018.5.18 (the current latest version) will be installed along with the core requirements
• If pipenv.enabled is True, whilst setting up the python environment, after installing the user requirements and before installing the package, the pipenv install --system {pipenv.options} command will be run.

Notes

• I have a patch ready for the readthedocs-build repository to go along with this one, should the green light be given here.
• As of right now, I have not written documentation for this feature, I wanted to get some feedback on this implementation first 😃

Thank you maintainers for the work you put into this wonderful service ❤️

[agjohnson]

Thanks for the patch! I'm excited to get pipenv support merged. I'll follow up on the #3181, as there is already some discussion there.

As per conversation there, I'm going to set this to blocking as we discuss the changes there a little more. Also, we're blocked on the port of readthedocs_build to this code base, so we don't complicate the porting.

[agjohnson]

So we've discuessed this more in #4354, I think we're aiming for a different schema that ties together a few more pieces. I think much of this should translate, but the schema will change fairly heavily.

@Tobotimus are you still looking to wrap up this PR? If not, @stsewd can continue this PR and put together the new schema chagnes.

[Tobotimus]

@agjohnson Unfortunately these days my schedule's a bit too tight to finish this up right now, so you're welcome to do what you will 😃 Obviously these commits can be included in a subsequent PR if so desired.

[stsewd]

@Tobotimus thank you! Sorry we came up with a decision too late.

[ericholscher]

Closing in favor of #4783 -- thanks for the contributions here!