Elasticsearch integration

Hello there,

I had to use monolog 2.3 to log to elasticsearch and I saw that there is no support for the engine.

This is a first implementation of it with the following sample configuration 

``` yaml
my_handler:
  elasticsearch:
      host: 127.0.0.1
  type: elasticsearch
  level: info
  channels: payments
  tags: monolog.logger
```

Let me know if you like and if you would like any changes. 

Please advice in case this should go to a different branch.

Cheers