Added Plugin: Automatic Default Domain

Hi,

This plugin is used to automatically set de default domain based on where the webmail is accessed. So if somone access the webmail via: webmail.example1.com. example1.com is set as the default domain. If someone else access the same installation via anotherdomain.com, the domain anotherdomain.com is set as default domain. So it's dynamic

Could you merge this? Would be much appreciated

Lennard