Add support for header comments

[emersion]

[yogo1212]

it appears that this is a frequent problem with recent versions of docker-mailserver (postfix + opendkim).

this is a typical Authorization-Results header value for dkim:

mail1.a.local; dkim=pass (2048-bit key; unprotected) header.d=b.local header.i=@b.local header.a=rsa-sha256 header.s=DKIM001 header.b=gBRN3GEb; dkim-atps=neutral

maybe @konimarti can bring #53 up to speed again?