Add option type_name_match_at_type to ESLogstashV0Encoder

So that the default logstash v0 behaviour of @type being the same as the index _type is achievable.

Discussed in #1096 
Hrm. I appreciate that you went to pains to not break backwards compatibility, but I actually think it probably makes more sense to default to match the Logstash V0 behavior. I'd say we should call  this `use_message_type`, like so:

- use_message_type (bool):

    If false, the generated JSON's @type value will match the ES record type specified in the `type_name`
    setting. If true, the message's Type value will be used as the @type value instead. Defaults to false.

Then we'll also want to add an entry to the CHANGES.txt changelog file recording this as a breaking change for the 0.8 release.