Support for adoc

[dersvenhesse]

I'd really appreciate support for AsciiDoc files with .adoc with UTI dyn.ah62d4rv4ge80c3dtqq. Rendering would be possible using bytesparadise/libasciidoc.

[samuelmeuli]

Cool library. The HTML conversion part is probably easy because it's written in Go and it also uses Chroma for syntax highlighting. This will allow us to add it to HTMLConverter.

It's very important to me that Glance's different previews looks consistent and that stylesheets are shared wherever possible. I think getting the CSS right will be the main challenge of this feature. Let's revisit this once bytesparadise/libasciidoc#63 has been implemented.