Support author attribute

[josevalim]

It will be the author for epubs and use the relevant author tag for HTML: https://www.metatags.org/meta_name_author

/cc @msaraiva

[whatyouhide]

@josevalim do we need to support this at the module level as an actual doc attribute, or do you mean just using the author in the package config?

[josevalim]

@whatyouhide in the package config. This will be for the "whole artifact".