Support _routing meta-field

[cdunn]

Add a new optional protocol DocumentMeta that adds the ability to specify a _routing key. This allows specific shard targeting which is required for join fields.

• https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-routing-field.html
• https://www.elastic.co/guide/en/elasticsearch/reference/current/parent-join.html

@danielberkompas let me know if you think this is the right approach and I can add a few more tests.

[danielberkompas]

@cdunn thanks for this! I'm wondering if we could just incorporate a new routing function into the existing Document protocol, rather than creating a new protocol.

defimpl Elasticsearch.Document, for: MyApp.Type do
  # if you don't want to use routing, just return false
  def routing(_document), do: false

  # if you want to use routing, do this:
  def routing(document) do
    # ...
  end
end

[cdunn]

@danielberkompas sounds good. I agree it's cleaner as a single protocol...I originally tried that but couldn't figure out if there was a way to make just the routing function fallback_to_any and false it there by default so it wouldn't be a breaking change given that most folks probably don't use it to start. Will consolidate and document later but let me know if you can think of any way to make this non-breaking.

[danielberkompas]

I don't think it matters too much if it's a breaking change. The library is still <1.0.0 at this point, so breaking changes are not a really big problem. As long as we write a good upgrade guide, I think it will be fine.

[coveralls]

Coverage increased (+0.08%) to 97.949% when pulling 504c926 on cdunn:routing into a8d5d34 on infinitered:master.

[danielberkompas]

This looks great, thanks! I'll try and get a new version released over the weekend.