Allow sending custom headers

[MultiSuperFreek]

When adding headers to the $headers array in the wp_mail filter, only a few recognized headers ('Content-Type', 'Cc', 'Bcc', 'Reply-To', 'From', 'X-PM-Track-Opens', 'X-PM-TrackLinks', 'X-PM-Tag') are picked up by the plugin, and the rest is just silently dropped. This PR adds headers that are not recognised now (and used by the plugin internally) to a custom headers array that is sent to the Postmark API, and thus end up in the message sent by Postmark.