Add support for asciidoctor

Thanks for the patch, I verified it still works with asciidoc binary.

The patch looks good, except for two minor things:

+  if ($is_command_avaiable) {
     $severity = NULL;
     $version = substr(array_shift($output), 9);
   }

The admin/reports/status page shows asciidoc version parsing output and assuming it is like asciidoc 8.6.9, you may want to change the substr(array_shift($output), 9); line to report the version correctly.

+  $is_command_avaiable = FALSE;

Typo.

I really have never used asciidoctor, but it looks like it supports the same arguments, so it should be OK to add it here.

Possible Future changes

One thing that would be great would be a way to choose among the two if the two are installed.
With the patch it chooses the first it finds prioritizing asciidoc binary, so behaviour is maintained, that's good.
I think the right place to do that is on the filter configuration, currently is it empty.

@hgoto Do you want to open a follow-up issue about that?

Thanks for the changes, this is now part of 8.x-1.x.

I did one minor change after adding it. See the following change notice: Removed REQUIREMENT_INFO/REQUIREMENT_OK from most hook_requirements() items for details.