add custom environment prefixing

As requested multiple times and finally written down in issues #13 and
 #14, the ability to specify a custom prefix for environment variable
parsing has been added.  It even allows the prefix to be set per
environment variable with a more sensible default stripping of argv[0].

Should fix #14.
Add environment variable expansion

Environment variables will now be expanded if they contain other
environment variables.  This is necessary for integration with Docker's
linking capability during environment variable composition.