Support locally cached packages for installation

[kumar303]

When running peep install ... it would be nice to specify a local directory where it can look for cached packages. This would help a lot in rebuilding Docker images since site-packages is not kept around.

Relates to issue #14 but might be a little simpler.

[kumar303]

Before pip changed, a patch was in the works here https://github.com/erikrose/peep/pull/61