Parse configuration for inter-container links.

Fixes #201.

Unlike the suggested configuration in #91, this doesn't include the remote application name, but just the remote port is is listening on. This isn't user friendly solution, but it is easier to implement and enough to be usable.
Given that pyyaml gives us `bytes` or `unicode` depending on whether the string is ascii-only. I don't think checking `StringTypes` here is a bug (but not normalizing is #636).