Configurable index settings

We currently assume that for the most part a deployer sets up global Elasticsearch index settings (for things like shards, replicas). Since searchlight manages its own indices, and since we don't want to rule out the possibility of running on a shared cluster, we should allow settings to be specified in config. There are a finite number that we might want to expose so it's probably ok to do so by name; a good start would be index.number_of_shards and index.number_of_replicas.