Retain search query highlight on cursor move

@guidovansteen discovered inconsistent behaviour when
zsh-syntax-highlighting is not sourced prior to
zsh-history-substring-search.

https://github.com/zsh-users/zsh-history-substring-search/issues/9

@sunaku wrote this patch.

plugins/history-substring-search/history-substring-search.zsh

@@ -89,11 +89,14 @@ zmodload -F zsh/parameter
 #
 if [[ $+functions[_zsh_highlight] -eq 0 ]]; then
   #
-  # Dummy implementation of _zsh_highlight()
-  # that simply removes existing highlights
+  # Dummy implementation of _zsh_highlight() that
+  # simply removes any existing highlights when the
+  # user inserts printable characters into $BUFFER.
   #
   function _zsh_highlight() {
-    region_highlight=()
+    if [[ $KEYS == [[:print:]] ]]; then
+      region_highlight=()
+    fi
   }
 
   #