YAML STRUCTURED AUTHOR SUPPORT IN HTML

added support within the default HTML template for structured author
details (author.name, author.affiliation, author.email) in the YAML
metadata block. Should match the example YAML metadata block on pandoc
website.
ADD CLASSNAMES 'author_afil' AND 'author_email'

after adding support for including YAML author.affiliation and author.email metadata values in the defatul.html template, I realized users may want to use custom css styles for these elements. this should allow that.
Merge branch 'feature/support-yaml-author-details' of https://github.com/jmlondon/rmarkdown into jmlondon-feature/support-yaml-author-details