Plugin: allow overriding editor options (tab size, etc)

[BrianGenisio]

Graphiql currently supports a theme editor override with editorTheme but I'm hoping to have finer control over the CodeMirror options. Ideally, a editorOptions property would allow any property to be overridden so that as a user, I can control things like the tab spacing:

editorOptions: {
    tabSize: 4,
    indentUnit: 4
}