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.

lib/misc.zsh

@@ -38,5 +38,7 @@ if [[ -z "$LC_CTYPE" && -z "$LC_ALL" ]]; then
 	export LC_CTYPE=${LANG%%:*} # pick the first entry from LANG
 fi
 
-# recognize comments
 setopt interactivecomments
+setopt auto_cd
+setopt multios
+setopt cdablevars