Add vendor library management support to pip

[ianb]

Do a lookup for config files, looking from the current directory for pip.conf/.pip.conf/pip.ini
Do some limited %() substitution in config files
Handle empty lines in config files better

This is a refreshing of #491 (with bad commits removed)

Original description:

This pull request makes pip look in the current and parent directories for pip.conf and loads it. Also %(here)s gets substituted in config values.

The motivation here is to make directory-local vendor libraries easier, with a config file like:

[install]
install_option = --install-purelib=%(here)s/vendor
    --install-platlib=%(here)s/vendor-binary
    --install-scripts=%(here)s/vendor/bin

Or something like that... the idea isn't complete. Feedback appreciated.

[ianb]

The second commit adds a new option which can be used to facilitate the creation of scripts that use vendor libraries

[pnasrat]

This generally looks good to me, but I'd like to have some tests anyway.

[avinoamr]

@pypa this would be a great addition as it will make the maintenance of self-encapsulating projects easier. Can you please update on the status of this pull-request? I'd love to help fill in the gaps if you think anything's missing.

[pnasrat]

@avinoamr as before it needs tests - it also needs to apply cleanly if you want to take that up feel free.

[dstufft]

I'm going to close this, It's been awhile and it has no active work being done on it. I'm also not entirely sure that it's a good idea in general.