Multi-arch support

[nopid]

A proposal to fix #2:

• go.mod to track dependencies
• a fix to take care of github.com/Sirupsen/logrus with go.mod
• update to src/bridge_utils.go to take into account IPv6 compatible libnetwork/iptables
• proper go cross-compilation (now you can compile the plugin for every arch even under macOS)
• invoke the Makefile with an ARCH argument to generate the plugin for a given architecture
• plugin version is now the architecture (kathara/katharanp:amd64, kathara/katharanp:arm64, etc)

Tested to build ARM64 and AMD64 versions, see https://hub.docker.com/r/nopid/katharanp/tags?page=1&ordering=last_updated