ex_docs with mermaid support

mix.exs

  def project do
    [
    ...
      docs: docs(),
    ...
    ]
  end

  defp docs do
    [
    ...
      assets: mermaid_assets(),
      before_closing_body_tag: mermaid_snippet(:file)
    ]
  end

  @doc """
  Returns the relative or absolute path which has the mermaid.min.js file.

  Fetch a copy of mermaid (from: https://mermaid-js.github.io/mermaid/)
  and add it to the local _assets_ directory.
  """
  def mermaid_assets() do
    "assets"
  end

  @doc """
  Returns a function that, when called, will return a html snippet with the correct file or
  url to load mermaid.
  """
  def mermaid_snippet(file_or_url) do
    url = case file_or_url do
    :file ->
      "assets/mermaid.min.js"
    url ->
      url
    end

    fn (:html) ->
      """
      <script src="#{url}"></script>
      <script>mermaid.initialize({startOnLoad:true});</script>
      """
       (_) -> ""
    end
  end
end