Allow to use callback to extends usage of notifier.

Signed-off-by: crynobone <crynobone@gmail.com>
Append subject if it’s provided.

Signed-off-by: crynobone <crynobone@gmail.com>
Rename to RecipientInterface to better illustrate the usage.

Signed-off-by: crynobone <crynobone@gmail.com>
Merge branch '2.1'