Add X-Git-Host header for better filtering

For setups with a host with many different git repositories there
was no practical way to filter "all git-multimail mails from that
host". The new X-Git-Host Header supports it - it contains the
fqdn of the host that generated the mails.

Signed-off-by: Malte Swart <mswart@devtation.de>
Change fqdn into a COMPUTED_KEY.

This allows it to be overridden by derived classes.
Add classes to allow FQDN to be configured manually.
Merge branch 'x-git-host-header'

Add a header "X-Git-Host", containing the fully-qualified domain name
of the server, as another bit of data that can be used for filtering
emails.