Support "kubectx NAME=." to rename current-context

Fixes #24.

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

kubectx

@@ -30,6 +30,7 @@ USAGE:
   kubectx <NAME>            : switch to context <NAME>
   kubectx -                 : switch to the previous context
   kubectx <NEW_NAME>=<NAME> : rename context <NAME> to <NEW_NAME>
+  kubectx <NEW_NAME>=.      : rename current-context to <NEW_NAME>
   kubectx -h,--help         : show this message
 EOF
   exit 1
@@ -122,6 +123,10 @@ rename_context() {
   local old_name="${1}"
   local new_name="${2}"
 
+  if [[ "${old_name}" == "." ]]; then
+    old_name="$(current_context)"
+  fi
+
   # TODO(ahmetb) old_user and old_cluster are no longer used, clean up
   local old_user old_cluster
   old_user="$(user_of_context "${old_name}")"