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

test/plugins/cmd-returned-notify.plugin.bats

@@ -0,0 +1,47 @@
+#!/usr/bin/env bats
+
+load ../test_helper
+load ../../lib/helpers
+load ../../lib/composure
+
+load ../../plugins/available/cmd-returned-notify.plugin
+
+@test "plugins cmd-returned-notify: notify after elapsed time" {
+    export NOTIFY_IF_COMMAND_RETURNS_AFTER=0
+    export LAST_COMMAND_TIME=$(date +%s)
+    sleep 1
+    run precmd_return_notification
+    assert_success
+    assert_output $'\a'
+}
+
+@test "plugins cmd-returned-notify: do not notify before elapsed time" {
+    export NOTIFY_IF_COMMAND_RETURNS_AFTER=10
+    export LAST_COMMAND_TIME=$(date +%s)
+    sleep 1
+    run precmd_return_notification
+    assert_success
+    assert_output $''
+}
+
+@test "plugins cmd-returned-notify: preexec no output" {
+    export LAST_COMMAND_TIME=
+    run preexec_return_notification
+    assert_success
+    assert_output ""
+}
+
+@test "plugins cmd-returned-notify: preexec no output env set" {
+    export LAST_COMMAND_TIME=$(date +%s)
+    run preexec_return_notification
+    assert_failure
+    assert_output ""
+}
+
+@test "plugins cmd-returned-notify: preexec set LAST_COMMAND_TIME" {
+    export LAST_COMMAND_TIME=
+    assert_equal "${LAST_COMMAND_TIME}" ""
+    NOW=$(date +%s)
+    preexec_return_notification
+    assert_equal "${LAST_COMMAND_TIME}" "${NOW}"
+}