* Rename middleware to plugin
first pass; mostly used 'sed', few spots where I manually changed
text.
This still builds a coredns binary.
* fmt error
* Rename AddMiddleware to AddPlugin
* Readd AddMiddleware to remain backwards compat
This fix adds unit tests for `ExtractAddressFromReverse`. It also
updates the implementation of `ExtractAddressFromReverse` so that
only a valid IPv4 or IPv6 address (in string format) is returned.
If the addresses is not valid and can not be handled by golang's
`net.ParseIP` then an empty string (`""`) will be returned.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>