resolves #362 use neutral attribute prefixes in AsciiDoc integration

- use page- prefix instead of awestruct- prefix for front matter
- retain support for awestruct- prefix, but consider deprecated
- write page variables to attributes prefixed with page-
- write selected site variables to attributes prefixed with site-
Merge pull request #363 from mojavelinux/issue-362

resolves #362 use neutral attribute prefixes in AsciiDoc integration