Function takes an anchor (and optional link text and formatting style)
and transforms the input into a link to an anchor id.
Currently works for markdown and wikilink style link transforms.
Options were previously set ad-hoc in the initialize function, but
re-set and overwritten at various places.
Options now live in one central module (options.lua), where they
directly access the neovim options set, and re-access them on any run.
That means options can be changed while the plugin is running and it
will use their newer versions, without requiring any re-initialization.