Add ability to switch between 'random' themes and 'favorite' themes

- Refactored code for getting a random theme into a function
- Setting your theme to 'favorites' will now pick a random theme from your
  favorites and setting theme to 'random' will pick a random theme from ALL
    available themes

lib/functions.zsh

@@ -78,3 +78,13 @@ function unlike_theme() {
         echo "$theme_name isn't a favorite"
     fi
 }
+
+function load_random_theme() {
+    themes=($*)
+    N=${#themes[@]}
+    ((N=(RANDOM%N)+1))
+    RANDOM_THEME=${themes[$N]}
+    source "$RANDOM_THEME"
+    echo "[oh-my-zsh] Random theme '$RANDOM_THEME' loaded..."
+    export RANDOM_THEME
+}