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/xterm.plugin.bats

@@ -0,0 +1,54 @@
+#!/usr/bin/env bats
+
+load ../test_helper
+load ../../lib/composure
+
+load ../../plugins/available/xterm.plugin
+
+function local_setup {
+  setup_test_fixture
+
+  # Copy the test fixture to the Bash-it folder
+  if command_exists -v rsync
+  then
+    rsync -a "$BASH_IT/test/fixtures/plugin/xterm/" "$BASH_IT/"
+  else
+    find "$BASH_IT/test/fixtures/plugin/xterm" \
+      -mindepth 1 -maxdepth 1 \
+      -exec cp -r {} "$BASH_IT/" \;
+  fi
+}
+
+@test "plugins xterm: shorten command output" {
+    export SHORT_TERM_LINE=true
+    run _short-command ${BASH_IT}/test/fixtures/plugin/xterm/files/*
+    assert_success
+    assert_output ${BASH_IT}/test/fixtures/plugin/xterm/files/arg0
+}
+
+@test "plugins xterm: full command output" {
+    export SHORT_TERM_LINE=false
+    run _short-command ${BASH_IT}/test/fixtures/plugin/xterm/files/*
+    assert_success
+    assert_output "$(echo ${BASH_IT}/test/fixtures/plugin/xterm/files/*)"
+}
+
+@test "plugins xterm: shorten dirname output" {
+    export SHORT_TERM_LINE=true
+    run _short-dirname
+    assert_success
+    assert_output "$(basename $PWD)"
+}
+
+@test "plugins xterm: full dirname output" {
+    export SHORT_TERM_LINE=false
+    run _short-dirname
+    assert_success
+    assert_output $PWD
+}
+
+@test "plugins xterm: set xterm title" {
+    run set_xterm_title title
+    assert_success
+    assert_output $'\033]0;title\007'
+}