Add Mobiledoc version support to mobiledoc parser and renderer

[mixonic]

We're shortly to introduce atoms and v0.3 of the mobiledoc format in #226. To ensure we have a smooth iteration path for these changes, the mobiledoc parser and renderer should be able to support multiple versions.

[bantic]

Somewhat related: bustle/mobiledoc-dom-renderer#13 and bustle/mobiledoc-html-renderer#10. The dom and html renderers will currently throw with a useful error message if they encounter an unknown version of mobiledoc (they only know about v 0.2.0 at the moment)

[bantic]

fixed by #241