Allow for customizing links

[chrispcampbell]

We should make it possible to customize the way Markdown links are generated into HTML. This is useful for defining custom link syntax, similar to the built-in support for glossary terms. For example, we could define links to model equations and have them appear differently in the generated HTML:

This is a link to a [model equation][mdl].

This requires adding support for a .config.js file (in addition to the existing .config.json), since the JS one allows for defining a handler function for links (and other element types eventually).