Add the ability to specify custom repos

[StephenAmar]

[benley]

Thank you for the pull request! I've been on vacation and didn't notice this PR until just now.

Is pex_repos something that should be set separately for each pex_binary/library/test rule, or would it make more sense to have it be configurable repo-wide in the WORKSPACE file, like how the maven_server repository rule works?

I have been hoping to evolve the pex rules toward letting Bazel take care of the downloading (and caching!) rather than having pex do it each time a pex_binary rule gets built. I haven't yet come up with a good way of accomplishing that, though...