Add version to plugin

version is set via ldflags at compile time. It should be JSON encoded
plugin.VersionType. If it does not unmarshal, the plugin version will be
left empty.

An example:
go build -ldflags '-X main.version={"Major":99,"Minor":88,"Build":77}'

main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"encoding/json"
 	"log"
 	"os"
 
@@ -30,9 +31,20 @@ func (c CFSyslogCLI) Run(conn plugin.CliConnection, args []string) {
 	}
 }
 
+// version is set via ldflags at compile time. It should be JSON encoded
+// plugin.VersionType. If it does not unmarshal, the plugin version will be
+// left empty.
+var version string
+
 func (c CFSyslogCLI) GetMetadata() plugin.PluginMetadata {
+	var v plugin.VersionType
+	// Ignore the error. If this doesn't unmarshal, then we want the default
+	// VersionType.
+	_ = json.Unmarshal([]byte(version), &v)
+
 	return plugin.PluginMetadata{
-		Name: "CF Syslog CLI Plugin",
+		Name:    "CF Syslog CLI Plugin",
+		Version: v,
 		Commands: []plugin.Command{
 			{
 				Name:     "drains",