Forward pip options from the exported requirements file

[cjolowicz]

We may want to collect pip options like --index-url from the exported requirements.txt, and pass them to pip via the command-line. Currently, these are simply filtered out, since they are not valid requirements.

(Are they valid in a constraints file? If that were to be the case, we could simply exclude them from the requirements parser.)

This came up in #397 and #497.

[raddessi]

I would spend some time on this if you think it is doable.

[raddessi]

OK! Not only is this doable It's not too big of a lift. I should have something for you to check out soon I hope.