Allow markdown-it plugins

[nmokkenstorm]

First of all, thanks for the work on this, I'm using the vim plugin on a daily basis, both professionally and on hobby projects.

As mentioned in #34 and #15, people seem to have a wish for being able to preview markdown with extensions. While I'm personally not really a fan of those, a project I'm working on right now uses a lot of PlantUML for documentation.

I personally dislike having to leave my terminal for something trivial like editing some UML diagrams, so I'd like to have the PlantUML plugin available within Livedown. Hardcoding this here wouldn't make a ton of sense as most people probably won't use this.

I'd be willing to look into creating a PR to allow for plugin support, if that would be appreciated. I'd love to discuss the option here, so please let me know what you think.