move history-substring-search* files into plugins/

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

@@ -0,0 +1,12 @@
+# This file integrates the history-substring-search script into oh-my-zsh.
+
+source "$ZSH/plugins/history-substring-search/history-substring-search.zsh"
+
+if test "$CASE_SENSITIVE" = true; then
+  unset HISTORY_SUBSTRING_SEARCH_GLOBBING_FLAGS
+fi
+
+if test "$DISABLE_COLOR" = true; then
+  unset HISTORY_SUBSTRING_SEARCH_HIGHLIGHT_FOUND
+  unset HISTORY_SUBSTRING_SEARCH_HIGHLIGHT_NOT_FOUND
+fi