Add email notifications to Domain Mapping plugin

It would be really handy to have an automatic email notification sent out to the super admin of the multisite after a member clicks the "Map domain" button.

Some hosting requires personal domain names to be added to the domain control in the hosting control panel manually in order for mapping to work. As is the case with our hosting.

However currently the super admin has to rely on members to contact support after they have failed to get it working.

It would be nice to have the option to enable a notification email alerting the super admin about a new domain being mapped and what that domain is.