Add Support for regexp Filter

[kylebrandt]

It looks like this filter isn't implemented:

http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/query-dsl-regexp-filter.html

Would that be possible? I'm trying to create something like the following (and regexp lets me deal with both numbered and string fields):

POST logstash-2015.01.08/_search?count=true
{
   "aggregations":{
      "g_eventlog_id":{
         "aggregations":{
            "g_TargetUserName":{
               "aggregations":{
                  "ts":{
                     "date_histogram":{
                        "field":"@timestamp",
                        "interval":"30m"
                     }
                  }
               },
               "terms":{
                  "field":"TargetUserName"
               }
            }
         },
         "terms":{
            "field":"eventlog_id"
         }
      }
   },
   "query": {
       "filtered":{
          "query": { 
              "range": {
                    "@timestamp":{
                        "from":"2015-01-08T19:37:59.091203948Z",
                        "include_lower":true,
                        "include_upper":true,
                        "to":"2015-01-08T20:37:59.091203948Z"
                    }
                }
          },
          "filter": {"regexp":{ "eventlog_id": "4624"}}
       }
   }
}