Add custom mappings and settings to defaults instead of replacing them

I needed to define a custom analyzer for one of my indexes along with a custom mapping for one field to use that analyzer. However, I didn't want to replace the default settings and mappings provided by Searchkick (because they're a pretty great set of defaults).

This change modifies the indexing behavior so that custom-defined mappings and settings are *added to* the default set instead of replacing them outright. I realize that this may be a breaking change but felt that it was important for anyone doing extensive customization -- or at least for us, anyway!

Thoughts / discussion welcome.

See https://github.com/ankane/searchkick/issues/99 for the original issue.