Support for Asciidoctor style of links

[mgoral]

This implements Asciidoctor Inter-Page Cross References style links:

<<file.adoc#,title>>

Implemented things:

• ordinary links things (creating, following, regular expresssions for parsing etc.)
• links toggling (wiki -> md -> adoc -> wiki)
• omni function (without anchors; regular expression for finding anchors is hardcoded in get_anchors function and creating a single regex for Asciidoc, next to g:wiki#rx#header_items feels hacky)

Fixes #109

[mgoral]

I think this is now more-less complete. @lervag, can you have a look?

[lervag]

Thanks! This was IMHO a very high quality PR, I just made some simple adjustments of the docs and merged it immediately. I did not test very thoroughly, and I think the only thing that could be said to be missing was one or two simple tests. I'll add that if I find it necessary myself.