Add support for X-Unsent header

Please add support for the X-Unsent header when loading .eml files.  When the X-Unsent header is 1 (X-Unsent:1), the .eml should be loaded in compose mode, so the user can add or modify the email and then click send.  This is useful for templates and integration with other programs.  There are other methods (for example MAPI) for loading a message in compose mode, but .eml is much simpler to implement and more stable from our experience.  Another option, mailto: does not support preloading a message with an attachment.

We have implemented this feature and it will be available in one of the upcoming minor releases.