handle explicit author attributes

[jaredh159]

see snippet below. when the parser encounters a document attribute trying to set author and/or email, we need to special case so that we can produce/parse an Author struct. these explicit attrs (as opposed to using the Author line) are documented to be only supported in the document header, so we can contain the logic there.

note in the snippet below how author_1 value shows up as John M Doe -- which sort of proves this exact example is really an error (maybe should produce a warning?) but at the very least explicitly setting via attrs (not combined with an author line) is a supported mechanism and needs to work.

see https://docs.asciidoctor.org/asciidoc/latest/attributes/document-attributes-ref/#document-metadata-attributes

= Doc Title
john M Doe <john@doe.com>; Bob Smith <bob@smith.org>
:author: Sally McAttr
:email: [email protected]


author={author}

author_1={author_1}

authors={authors}

email={email}

initials={authorinitials}

[subs=-macros]
email={email}