Themes: Fold themes plugin in to core oh-my-zsh

Rewrite theme() and lstheme() to remove side effects, add debugging,
and fix `local` inside theme definitions.
Add `theme next` for stepping through theme list, and "blacklist" support.
Modify tools/theme_chooser to use new theme functions.

plugins/themes/_theme

@@ -1,3 +0,0 @@
-#compdef theme
-
-_arguments "1: :($(lstheme | tr "\n" " "))"

plugins/themes/themes.plugin.zsh

@@ -1,24 +0,0 @@
-function theme
-{
-    if [ -z "$1" ] || [ "$1" = "random" ]; then
-	themes=($ZSH/themes/*zsh-theme)
-	N=${#themes[@]}
-	((N=(RANDOM%N)+1))
-	RANDOM_THEME=${themes[$N]}
-	source "$RANDOM_THEME"
-	echo "[oh-my-zsh] Random theme '$RANDOM_THEME' loaded..."
-    else
-	if [ -f "$ZSH_CUSTOM/$1.zsh-theme" ]
-	then
-	    source "$ZSH_CUSTOM/$1.zsh-theme"
-	else
-	    source "$ZSH/themes/$1.zsh-theme"
-	fi
-    fi
-}
-
-function lstheme
-{
-    cd $ZSH/themes
-    ls *zsh-theme | sed 's,\.zsh-theme$,,'
-}