add --scheme argument

[greyltc]

solves #120

This adds a new optional argument, --scheme, that will force the installation to be done using a specific scheme instead of the default one.

example usage:

python -m installer --scheme posix_user some.whl

see https://docs.python.org/3/library/sysconfig.html?highlight=installation%20scheme#installation-paths
for a list of valid schemes

[greyltc]

Is there anything more I can do to help get this merged?

[pradyunsg]

Not really -- I've got this on my review queue for tomorrow. :)

[greyltc]

The most obvious (to me) thing that's missing here is documentation. argparse's --help should do a fine job of explaining the new option, but I haven't touched anything in docs/

[pradyunsg]

Hmm... this seems be related to #145 as well; which is the direction I'd prefer to go TBH -- bringing over the existing "battle-tested" code from pip into this library/CLI and exposing it.

[pradyunsg]

So... I think going down the route of #145 will either supercede this or be a superset of this change -- and making progress toward that seems like a more sustainable way to evolve overall ("use the same logic as pip" has a strong appeal to it).

To that end, I think I'll close this out -- not because this isn't a complete and correct change on it's own (thanks @greyltc for doing this, being patient and making this really easy to review!) but because I'd like to avoid adding this to the public CLI and restricting this project from potentially being locked out of the ability to pull in behaviours+logic from pip itself.