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.
Use of `StringTypes` here was a bug before, so use in the new code is still a bug.  Please file an issue for making this code only spit out `unicode` (or perhaps `bytes` since that's what a POSIX environment variable is actually made of... but I could believe the argument "utf-8 or gtfo").  Link to the issue from this code.