Add AsciiDoc support

Summary:
AsciiDoc syntax highlighting aiming to behave like Asciidoctor and
use colours similar to its default style sheet.

Reviewers: Framework: Syntax Highlighting, dhaumann

Reviewed By: Framework: Syntax Highlighting, dhaumann

Subscribers: dhaumann, nibags, kwrite-devel, kde-frameworks-devel, Framework: Syntax Highlighting

Tags: Framework: Syntax Highlighting, Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D18475