Merge pull request #2202 from kevinxucs/themes-random

themes plugin now picks a random theme if no argument is provided.

plugins/themes/themes.plugin.zsh

 function theme
 {
-    if [ "$1" = "random" ]; then
+    if [ -z "$1" ] || [ "$1" = "random" ]; then
 	themes=($ZSH/themes/*zsh-theme)
 	N=${#themes[@]}
 	((N=(RANDOM%N)+1))