Support sub-package managers (e.g. pip)

[pelson]

Tl;dr : support pip specifications of packages as well

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I received the following email:

I was wondering though if it's possible to include pip packages in the environment specification? For example, my environment.yml file looks like:

dependencies:
- python=3
- requests
- xlsxwriter
- pip
- pip:
    - dateparser==0.4.0
    - tweepy
    - ujson

[pelson]

Would be especially nice to be able to do fancy pip URIs such as git+https://github.com/...