Allow users to add their own markdown-it plugins

[msimmer]

b-ber would have to load the plugin and use it in bber-plugins/md/index.es

[msimmer]

i think the best solution for this might be to set sth up like the MD-it plugin loader here which would depend on having a node_modules directory in the project, register the plugins in the config.yml, and then have bber look in the node_modules dir to load the necessary plugins

-- edit

i think it's good to lean on npm for any type of extensions, since bber will share the same ecosystem, and bber's themes are already managed with npm

[msimmer]

This has been added to the roadmap project