'pip.commands' entry point for pip command plugins

this adds support for external projects adding pip commands using a new 'pip.commands' entry point.
pretty simple.  more could be done with the help formatting.
see the test for an example project using the entry point.

#1061 mentions doing this with "git style redirection" (with an example gist link).  the example was about execution. it was unclear to me how it provided a solution similar to entry points, but maybe I was missing something.