Support for non-major mobiledoc versions?

[rsoo]

Hey, thanks for your work on this!

Was wondering why there's only support for MOBILEDOC_VERSION 0.2.0 and 0.3.0?

# lib/mobiledoc_html_renderer.rb

def render(mobiledoc)
  version = mobiledoc['version']

  case version
  when '0.2.0', nil
    Renderer_0_2.new(mobiledoc, state).render
  when '0.3.0', nil
    Renderer_0_3.new(mobiledoc, state).render
  else
    raise Mobiledoc::Error.new(%Q[Unexpected Mobiledoc version "#{version}"])
  end
end

My use case involves Mobiledoc version 0.3.1 and I had to comment out the error and explicitly call the Renderer for 0.3.0

Curious -- what was the reasoning behind this? Would you like for me to open a PR to match any valid Mobiledoc version? (#12)