feat:Add hash to message link

Hi Johannes,

I'm using your tool since last year, and I think this is a great tool.

Sometimes I need to be able to be able to prove the authenticity of the attachment I linked to my message. This is the reason why I do not use your tool each time I need to share a file.

If the hash (MD5 for example) was written in the email, in the link part, this aim would be achieved, and I could use your tool every time, and promote your tool to every one.

What do you think about that please ?

-- Best regards, Jérôme, from France.