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

bash_it.sh

@@ -49,6 +49,17 @@ do
   fi
 done
 
+# Load vendors
+BASH_IT_LOG_PREFIX="vendor: "
+for _bash_it_vendor_init in "${BASH_IT}"/vendor/init.d/*.bash
+do
+  _log_debug "Loading \"$(basename "${_bash_it_vendor_init}" .bash)\"..."
+  # shellcheck disable=SC1090
+  source "${_bash_it_vendor_init}"
+done
+unset _bash_it_vendor_init
+
+BASH_IT_LOG_PREFIX="core: main: "
 # Load the global "enabled" directory
 # "family" param is empty so that files get sources in glob order
 # shellcheck source=./scripts/reloader.bash
@@ -62,7 +73,7 @@ do
 done
 
 # Load theme, if a theme was set
-if [[ ! -z "${BASH_IT_THEME}" ]]; then
+if [[ -n "${BASH_IT_THEME}" ]]; then
   _log_debug "Loading \"${BASH_IT_THEME}\" theme..."
   # Load colors and helpers first so they can be used in base theme
   BASH_IT_LOG_PREFIX="themes: colors: "