Add support for the "Delete by Query" API (#11)

This commit adds support for ElasticSearch's Delete by Query API. Usage
is very similar to that of Search and Count requests:

    esquery.Delete().
        Index("index_1, "index_2").
        Query(esquery.Bool()...).
        Run(
            es,
            esapi.WithAnalyzeWildcard(true),
        )

delete.go

@@ -0,0 +1,63 @@
+package esquery
+
+import (
+	"bytes"
+	"encoding/json"
+
+	"github.com/elastic/go-elasticsearch/v7"
+	"github.com/elastic/go-elasticsearch/v7/esapi"
+)
+
+// DeleteRequest represents a request to ElasticSearch's Delete By Query API,
+// described in
+// https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-delete-by-query.html
+type DeleteRequest struct {
+	index []string
+	query Mappable
+}
+
+// Delete creates a new DeleteRequest object, to be filled via method chaining.
+func Delete() *DeleteRequest {
+	return &DeleteRequest{}
+}
+
+// Index sets the index names for the request
+func (req *DeleteRequest) Index(index ...string) *DeleteRequest {
+	req.index = index
+	return req
+}
+
+// Query sets a query for the request.
+func (req *DeleteRequest) Query(q Mappable) *DeleteRequest {
+	req.query = q
+	return req
+}
+
+// Run executes the request using the provided ElasticSearch client.
+func (req *DeleteRequest) Run(
+	api *elasticsearch.Client,
+	o ...func(*esapi.DeleteByQueryRequest),
+) (res *esapi.Response, err error) {
+	return req.RunDelete(api.DeleteByQuery, o...)
+}
+
+// RunDelete is the same as the Run method, except that it accepts a value of
+// type esapi.DeleteByQuery (usually this is the DeleteByQuery field of an
+// elasticsearch.Client object). Since the ElasticSearch client does not provide
+// an interface type for its API (which would allow implementation of mock
+// clients), this provides a workaround. The Delete function in the ES client is
+// actually a field of a function type.
+func (req *DeleteRequest) RunDelete(
+	del esapi.DeleteByQuery,
+	o ...func(*esapi.DeleteByQueryRequest),
+) (res *esapi.Response, err error) {
+	var b bytes.Buffer
+	err = json.NewEncoder(&b).Encode(map[string]interface{}{
+		"query": req.query.Map(),
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	return del(req.index, &b, o...)
+}