Support for nested keys

[moroshko]

@markdalgleish I'd appreciate your thoughts on moroshko/react-autosuggest#301

Would it make sense to extend react-themeable to support nested keys in the input?

Or, maybe react-autosuggest should make sure that it can be styled using a flat theme (without nested keys) - this is obviously more limiting.

[theobat]

I think nested keys is a great idea too ! (i'm also a react autosuggest user) @markdalgleish any reason we shouldn't consider doing this ?