Add support for post filter in Search

Search requests can now accept a post filter applied after all hits were
returned from the database. For example:

    esquery.Search().
        Query(...).
        Aggs(...).
        PostFilter(esquery.Range(field).Gt(0)).
        Run(...)

search.go

@@ -15,12 +15,13 @@ import (
 // Not all features of the search API are currently supported, but a request can
 // currently include a query, aggregations, and more.
 type SearchRequest struct {
-	query   Mappable
-	aggs    []Aggregation
-	from    *uint64
-	size    *uint64
-	explain *bool
-	timeout *time.Duration
+	query      Mappable
+	aggs       []Aggregation
+	postFilter Mappable
+	from       *uint64
+	size       *uint64
+	explain    *bool
+	timeout    *time.Duration
 }
 
 // Search creates a new SearchRequest object, to be filled via method chaining.
@@ -40,6 +41,12 @@ func (req *SearchRequest) Aggs(aggs ...Aggregation) *SearchRequest {
 	return req
 }
 
+// PostFilter sets a post_filter for the request.
+func (req *SearchRequest) PostFilter(filter Mappable) *SearchRequest {
+	req.postFilter = filter
+	return req
+}
+
 // From sets a document offset to start from.
 func (req *SearchRequest) From(offset uint64) *SearchRequest {
 	req.from = &offset
@@ -81,6 +88,9 @@ func (req *SearchRequest) Map() map[string]interface{} {
 
 		m["aggs"] = aggs
 	}
+	if req.postFilter != nil {
+		m["post_filter"] = req.postFilter.Map()
+	}
 	if req.size != nil {
 		m["size"] = *req.size
 	}

search_test.go

@@ -42,6 +42,7 @@ func TestSearchMaps(t *testing.T) {
 					StringStats("tag_stats", "tags").
 						ShowDistribution(true),
 				).
+				PostFilter(Range("score").Gt(0)).
 				Size(30).
 				From(5).
 				Explain(true).
@@ -87,6 +88,13 @@ func TestSearchMaps(t *testing.T) {
 						},
 					},
 				},
+				"post_filter": map[string]interface{}{
+					"range": map[string]interface{}{
+						"score": map[string]interface{}{
+							"gt": 0,
+						},
+					},
+				},
 				"size":    30,
 				"from":    5,
 				"explain": true,