Mermaid stateDiagrans with fork join don't render

[chrisflatley]

Probably an issue upstream with remark / mermaidjs plugin, you can't use fork join in the stateDiagram.

Example (from the mermaidjs website)

```mermaid
 stateDiagram
    state fork_state <<fork>>
      [*] --> fork_state
      fork_state --> State2
      fork_state --> State3

      state join_state <<join>>
      State2 --> join_state
      State3 --> join_state
      join_state --> State4
      State4 --> [*]
```

I suspect this is something to do with the <<

Warning: The tag <undefined> is unrecognized in this browser. If you meant to render a React component, start its name with an uppercase letter.
    in undefined (created by MDXCreateElement)
    in MDXCreateElement (created by MDXContent)
    in wrapper (created by MDXCreateElement)
    in MDXCreateElement (created by MDXContent)
    in MDXContent (created by MDXRenderer)
    in MDXRenderer (at Markdown.tsx:30)
    in MDXProvider (at Markdown.tsx:12)
    in Markdown (at docs.tsx:16)
    in main (at Layout.tsx:84)
    in div (created by ForwardRef(Container))
    in ForwardRef(Container) (created by WithStyles(ForwardRef(Container)))
    in WithStyles(ForwardRef(Container)) (at Layout.tsx:83)
    in main (created by Content)
    in Content (at Layout.tsx:82)
    in div (created by Root)
    in Root (at Layout.tsx:59)
    in div (created by ThemeProvider$1)
    in ThemeProvider (created by ThemeProvider$1)
    in ThemeProvider$1 (at Layout.tsx:52)
    in Layout (at docs.tsx:15)
    in _default (created by HotExported_default)
    in AppContainer (created by HotExported_default)
    in HotExported_default (created by PageRenderer)
    in PageRenderer (at json-store.js:93)
    in JSONStore (at root.js:51)
    in RouteHandler (at root.js:73)
    in div (created by FocusHandlerImpl)
    in FocusHandlerImpl (created by Context.Consumer)
    in FocusHandler (created by RouterImpl)
    in RouterImpl (created by Context.Consumer)
    in Location (created by Context.Consumer)
    in Router (created by EnsureResources)
    in ScrollContext (at root.js:64)
    in RouteUpdates (at root.js:63)
    in EnsureResources (at root.js:61)
    in LocationHandler (at root.js:119)
    in LocationProvider (created by Context.Consumer)
    in Location (at root.js:118)
    in Root (at root.js:126)
    in MDXProvider (at wrap-root-element.js:65)
    in MDXScopeProvider (at wrap-root-element.js:64)
    in Unknown
    in Unknown (at wrap-root-element.js:72)
    in _default (at app.js:67)

[chrisflatley]

Similar issue in with classDiagrams, eg

classDiagram
  class Example 
    <<entity>>
    String variable
    int length()