refactor all history modifications into discrete plugins

plugins/available/history-substring-search.plugin.bash

@@ -0,0 +1,9 @@
+cite about-plugin
+about-plugin 'search history using the substring already entered'
+
+# enter a few characters and press UpArrow/DownArrow
+# to search backwards/forwards through the history
+if [[ ${SHELLOPTS} =~ (vi|emacs) ]]; then
+	bind '"\e[A":history-substring-search-backward'
+	bind '"\e[B":history-substring-search-forward'
+fi