Allow to pass pip dependencies to get-pip script

[anti-social]

Some other variants to name it:

• pip-dependencies
• get-pip-args

[tailhook]

This is interesting: wasn't that /non-existent from #553, because this allowed pip not install setuptools (and you could set your own version in requirements.txt)? Is there something simple that requires old setuptools that I can test with?

Although, I'm not against this solution even if the above is true. It maybe somewhat more straightforward approach.

Let's name it get-pip-args. But keep the example with setuptools in docs, so it's more or less clean what it is for.

[anti-social]

There are a lot of issues: https://github.com/pypa/setuptools/issues?q=is%3Aissue+use_2to3

[anti-social]

because this allowed pip not install setuptools (and you could set your own version in requirements.txt)

At the moment pip always installs its own setuptools into target path.

[tailhook]

Okay. Let's just rename it to get-pip-args as we don't validate them anyway.