Change flag for custom relay config location to `-c, --config`

[christophermaier]

From @vanstee on May 4, 2016 21:23

Copied from original issue: operable/cog#628

[ohaiwalt]

The standard library flag package won't support the GNU behavior as requested, going to put up a PR replacing it with github.com/spf13/pflag as a drop-in that is POSIX compliant.