Add new file for detecting exact version in Joomla (aggressive mode)

[juananpe]

Add '/administrator/manifests/files/joomla.xml' as a path that should be checked (if it exists, extract the matched version)

[urbanadventurer]

Thanks @juananpe for adding version detection with the /administrator/manifests/files/joomla.xml file!

[urbanadventurer]

@juananpe would you please the authors section of this plugin with your name, version and a short description of the update.