* Add NSID plugin support for CoreDNS

This fix adds NSID plugin support for CoreDNS, as was proposed
in 1256.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add test cases for NSID plugin
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Generate code for NSID plugin
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Use hostname as the default (as with bind), and remove unneeded copy
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add README.md
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>