Assign implicit file info attributes per page

[mojavelinux]

Assign the implicit file info attributes per page that would normally be set when invoking the AsciiDoc processor on the file. These attributes include the following:

• docfile - equivalent to resource.source_file
• docdir - the parent directory of resource.source_file
• docfilesuffix - the file extension of resource.source_file
• docname - the basename of resource.source_file minus file extension

We may also want to consider assigning the attributes regarding the document mtime, but these seem less important in the context of a site:

• docdate
• doctime
• docdatetime
• docyear

[mojavelinux]

I'm going to defer on the date stuff, leave that for another day. Middleman does not expose the mtime on the file descriptor, so it would mean we have to consult the filesystem and that just feels like it adds lag for no reason.