fenceparser and unist-util-visit should be dependency, not devDependency

Hi there,

Awesome plugin, I just started experimenting with it. 

I noticed though, that after installing it, it doesn't work, because `fenceparser` and `unist-util-visit` is not part of the build. I think they should be part of the dependencies.

If I have time, I add a pull request, but I wanted to have a note of it here.

Thanks,
Hunor