Ability to set body_options globally in the same way you can model_options

[schinery]

I've just discovered I need to add body_options: { track_total_hits: true } to a number of searches, and for my use case I would really like to set this once so it is used for all Searchkick queries rather than where each search is triggered.

Basically, be able do...

Searchkick.body_options = {
  track_total_hits: true
}

And have it merged with the body_options here when a search is done.

Happy to attempt a PR if you think this would be useful?