Proposal: overrides for installers

[sinoroc]

@pf_moore

@notatallshaw

I see pre-release specifiers used in the wild in three different ways that would not work well with the -pre flag Pip offers

Having some sort of per-project “allow prereleases” option (--allow-pre-for=numpy?) would be a perfectly reasonable thing to offer, and well within the spitit of what even PEP 440 says (i.e., it’s not a “versioning v2” type of change)

This seems like it would be good a fit for this idea/proposal.

In general, I think it would solve a bunch of issues if more installation options were selectable per dependency (instead of for the whole installation session).