Show all headers in output

Not sure if this goes against the spirit of this gem, but I found it useful to be able to see all the headers, specific custom ones my mailers are setting.  Needed to increase the `dt` size to fit headers with longer names.  The value 180 was arrived at entirely unscientifically.

If there's a way to make this configurable that I should know about please let me know and I can make it so.