Add index raw format option
 Relative to
 #19 and this commit https://github.com/jverelst/elasticsearch-river-twitter/commit/a613e69096e2639eadad75a2169ab66fca38da60 and to #13, we want to support raw tweet format indexing.

 New option `"raw" : false` (default to `false`).

 ```sh
 curl -XPUT localhost:9200/_river/my_twitter_river/_meta -d '
 {
     "type" : "twitter",
     "twitter" : {
         "user" : "twitter_user",
         "password" : "twitter_password",
         "raw" : true
     }
 '
 ```
 Closes #25.