Override index settings

In response to @heffergm request for https://github.com/pelias/schema/issues/6

This PR allows users to override `elasticsearch` settings config.
***note*** these changes only apply at index creation, so they need to be set before you create the index.

@fdansv this will also allow you to specify your own `analyser` config in a local `pelias.json` config without having to modify the default settings.

@hkrishna FYI