Use logging (#1718)

* update docs

* plugins: use plugin specific logging

Hooking up pkg/log also changed NewWithPlugin to just take a string
instead of a plugin.Handler as that is more flexible and for instance
the Root "plugin" doesn't implement it fully.

Same logging from the reload plugin:

.:1043
2018/04/22 08:56:37 [INFO] CoreDNS-1.1.1
2018/04/22 08:56:37 [INFO] linux/amd64, go1.10.1,
CoreDNS-1.1.1
linux/amd64, go1.10.1,
2018/04/22 08:56:37 [INFO] plugin/reload: Running configuration MD5 = ec4c9c55cd19759ea1c46b8c45742b06
2018/04/22 08:56:54 [INFO] Reloading
2018/04/22 08:56:54 [INFO] plugin/reload: Running configuration MD5 = 9e2bfdd85bdc9cceb740ba9c80f34c1a
2018/04/22 08:56:54 [INFO] Reloading complete

* update docs

* better doc

plugin.md

@@ -38,7 +38,15 @@ See a couple of blog posts on how to write and add plugin to CoreDNS:
 If your plugin needs to output a log line you should use the `plugin/pkg/log` package. This package
 implements log levels. The standard way of outputting is: `log.Info` for info level messages. The
 levels available are `log.Info`, `log.Warning`, `log.Error`, `log.Debug`. Each of these also has
-a `f` variant.
+a `f` variant. The plugin's name should be included, by using the log package like so:
+
+~~~ go
+import clog "github.com/coredns/coredns/plugin/pkg/log"
+
+var log = clog.NewWithPlugin("whoami")
+
+log.Info("message") // outputs: [INFO] plugin/whoami: message
+~~~
 
 In general, logging should be left to the higher layers by returning an error. However, if there is
 a reason to consume the error and notify the user, then logging in the plugin itself can be