Add support for --atx-headers in asciidoc and remove unnecessary [[]] generation for headers.

With this change running pandoc like this:

  $ pandoc -t asciidoc --atx-headers

and its output will use the '== Header level 2' format instead.

This patch is just about the functionality - i'm not sure how to add tests for this (i'm a haskel noob) thus if anyone could be helpful with some pointers that would be great.