Add imprint

[Pokerkoffer]

Hi, first of all, thanks for this great library.

Currently, the body can be customized via markdown, however this is not possible for copyright.

As required by law, you need the senders contact details (imprint) in the mail, it would be great when the copyright field can be used for that, or even create a new field.

Mostly they have the following structure:

Company Name
John Doe Street 11
<postalcode> Cityname

This will be transformed to Company Name John Doe Street 11 <postalcode> Cityname which is not that great. Thus, markdown or keeping the line breaks at least would be great. This may not work due to html sanitizing.

[Pokerkoffer]

I felt free and added a PR, i hope this is fine for you: #73