Add props to configure CodeMirror

[janmyler]

Enable passing configuration options for CodeMirror via props from <Playground /> while keeping the original defaults.

Adds: lineNumbers, smartIndent, lineWrapping, tabSize.

[Intregrisist]

I was just about to make a PR for this. I wonder if it might be better to make it an object prop:

<Playground
   editor={{
    lineNumbers: true
  }}
/>