* feat: allow hostname override for email notifier
As it currently stands all notifiers utilise `os.Hostname` to populate their titles/subjects.
When utilising Docker with a bridged network if you set the hostname for a container to an external DNS hostname Docker's internal DNS resolver will override said hostname for all containers within the bridged network.
This change allows a user to specify what hostname should be represented in the email notifications without having to change the `os.Hostname`.
* feat: allow custom hostname for all notifiers
* docs: adjust notification hostname flag
