Add label support for kubectl describe

Add -l option to kubectl describe command, allow it return
multiple results filtered by label selector.

contrib/completions/bash/kubectl

@@ -275,6 +275,8 @@ _kubectl_describe()
 
     flags+=("--help")
     flags+=("-h")
+    flags+=("--selector=")
+    two_word_flags+=("-l")
 
     must_have_one_flag=()
     must_have_one_noun=()