Adding preexec as a vendored library

Added a vendored lib loading routine in bash-it.sh
Added documentation on how to vendor libs in bash-it
Added and fixed plugins using preexec
Added tests for two plugins
Removed the old preexec lib

plugins/available/cmd-returned-notify.plugin.bash

@@ -0,0 +1,16 @@
+# shellcheck shell=bash
+cite about-plugin
+about-plugin 'Alert (BEL) when process ends after a threshold of seconds'
+
+precmd_return_notification() {
+	export LAST_COMMAND_DURATION=$(($(date +%s) - ${LAST_COMMAND_TIME:=$(date +%s)}))
+	[[ ${LAST_COMMAND_DURATION} -gt ${NOTIFY_IF_COMMAND_RETURNS_AFTER:-5} ]] && echo -e "\a"
+	export LAST_COMMAND_TIME=
+}
+
+preexec_return_notification() {
+	[ -z "${LAST_COMMAND_TIME}" ] && export LAST_COMMAND_TIME=$(date +%s)
+}
+
+precmd_functions+=(precmd_return_notification)
+preexec_functions+=(preexec_return_notification)