Custom directives support

[yhatt]

Add Marpit.customDirectives property to define custom directives per instance. It resolves #124.

This object has the assignable global and local object. By defining the parser function to each object, Marpit would recognize additional directives. The parser should return the validated object that includes new value(s) for updating meta of markdown-it token.

The built-in directive parsers cannot overload by custom directives. The name of built-in directives cannot assign as new meta value too.

Currently, we do not intend to document about custom directives at https://marpit.marp.app/, because it will only affect internal tokens.

We will use that in the future VSCode plugin of Marp. Marp slide deck would be able to preview when marp: true is defined in front-matter. See yhatt/marp#118.

const marpit = new Marpit()
marpit.customDirectives.global.marp = v => ({ marp: v === 'true' })

[yhatt]

UPDATE: Directives must parse for recognizing marp: true, but a few plugins are still working when VSCode's markdown-it has extended by marpit.markdownItPlugins. To prevent breaking other VSCode plugins, we will change the approach to use the independent plugin for parsing marp: true. We enhanced markdown-it plugin system in #127.

The custom directive is still valuable for Marp Core.