Refactor plugin to be composed of little broccoli plugins.

[billybonks]

Index.js now behaves like a stream configurator instead of a plugin that did all the things. we need a major bump since I deleted the deprecated code.

What are the gains?

1. Less processing since we don't process markdown files over and over again.
2. Better composability we could now drop in replace a ToC or a serializer.

What are the tradeoffs

I probably do a bit more IO, I could have done less but chose a cleaner solution, with a strong composability API

here is something I found while looking for in-memory options. broccolijs/broccoli#355.

Next steps

we can open up new API options.
need to do a bit more discussion around toc, (serialization is coupled with construction).
should we move the plugins out of the repo?

[mansona]

I am supper happy with this work and I'm really looking forward to pushing the merge button 🎉