Added (OS specific) clipboard implementation.

- Based on github.com/atotto/clipboard.

ui/clipboard/clipboard.go

@@ -0,0 +1,21 @@
+package clipboard
+
+import (
+	clip "github.com/atotto/clipboard" // actual implementation
+	"opslag.de/schobers/zntg/ui"
+)
+
+type clipboard struct {
+}
+
+func (c clipboard) WriteText(t string) error {
+	return clip.WriteAll(t)
+}
+
+func (c clipboard) ReadText() (string, error) {
+	return clip.ReadAll()
+}
+
+func init() {
+	ui.DefaultClipboard = &clipboard{}
+}