Contributing the gtksourceview file to upstream

[msuchane]

I've taken the liberty of opening a merge request in the upstream gtksourceview project to add your asciidoc.lang file there. I hope that's alright with you -- if not, please let me know. I'd like your syntax highlighting rules to be available by default in editors.

The merge request: https://gitlab.gnome.org/GNOME/gtksourceview/merge_requests/60

This is really just to let you know and to hear your opinion.

Thanks for your work on this project!