Add an optional Polyglot.js-Options prop

[Charioteer]

I was in the need of using Polyglot.js' onMissingKey-callback to send missing translations automatically to my API. To do so i have added an optional polyglotOptions-prop in CoreAdmin to pass an additional settings object to use Polyglot's full features.

[fzaninotto]

This cannot be PRed against master, as it is a new feature, so please target next.

Also, we're doing our best to avoid adding more props to <Admin>. I would agree to support an options prop in the TranslationProvider component, but in order to use it, you'll have to use the Custom App approach, unless you find a more elegant way that does not involve adding a new prop to Admin.

[fzaninotto]

Superseded by #3685