Add option to the Send-MTMail function to only create and return the HTML body

[magnusjak]

As #407 I also want to be able to send e-mails outside of this built-in method. So I modified the Send-MTMail function to have a 'CreateBodyOnly' switch that basically skips the mail-send and just returns the body it creates.

This way I can re-use the amazing HTML-body you guys have created, and add this into my own way of sending e-mails. Or another choice would be to just create a separate function to create the body.

So people like me with this need don't have to manually update the module for each version released 😄