Autoload improvments

[gergondet]

This PR fixes three small issues with the plugin autoload feature:

• defaults autoload to OFF, one can pass the AUTOLOAD parameter to the add_plugin macro to restore the old behavior
• when the autoload option is switched off, the file is also uninstalled (previously it had to be removed manually)
• trim the content of the autoload file, this prevents an issue when the file is edited manually