Added windows executable extensions to Kubectl plugins

Currently, the kubectl plugins system - when running under Windows - will determine that a plugin is executable
by looking at its extension, comparing it to '.exe'.

I think we should allow plugins w/ a few more known extensions (.bat, .com, .cmd; for example) -
This would give more of a similar experience to plugins under *nix, where shell scripts are often used.

Moreover, there were two windows-specific behaviors that were redundant,
which I've removed:

1. In plugin.go - There's no need to differentiate between windows and
under OS-es when looking at the PATH variable; It can be refered as 'PATH'
in Windows as well

2. In cmd.go - There's no need to append a suffix when looking up
plugins on windows; One can safely use 'exec.LookPath' without a suffix
on Windows - and get automatic resolving for known executable types

pkg/kubectl/cmd/plugin/plugin.go

@@ -100,11 +100,7 @@ func (o *PluginListOptions) Complete(cmd *cobra.Command) error {
 		seenPlugins: make(map[string]string, 0),
 	}
 
-	path := "PATH"
-	if runtime.GOOS == "windows" {
-		path = "path"
-	}
-	o.PluginPaths = filepath.SplitList(os.Getenv(path))
+	o.PluginPaths = filepath.SplitList(os.Getenv("PATH"))
 	return nil
 }
 
@@ -230,7 +226,10 @@ func isExecutable(fullPath string) (bool, error) {
 	}
 
 	if runtime.GOOS == "windows" {
-		if strings.HasSuffix(info.Name(), ".exe") {
+		fileExt := strings.ToLower(filepath.Ext(fullPath))
+
+		switch fileExt {
+		case ".bat", ".cmd", ".com", ".exe":
 			return true, nil
 		}
 		return false, nil