New extension point in mailer-plugin exclude email recipients

This change simply adds an extension point to mailer-plugin that filters email recipients.

By default nothing is filtered. Also the mailer-plugin functionality is not extended to support any filtering.

This change allows writing simple plugins that make it possible to exclude certain email addresses
from emails sent by a Jenkins instance (e.g. system users or commiters to upstream projects)

Currently, implementing such a feature could be done using the mail-ext scripts. 
However, using mail-ext requires changes to job configurations and writing scripts
where a custom plugin that implements an ExtensionPoint would be more appropriate.

If this change makes it into mailer-plugin/master, a new plugin can be created that show cases
this feature (e.g. as implemented by mail-exts "excludedRecipients").