add support for custom scalars -- mainly adding proper import machinery

generate/testdata/queries/DateTime.graphql.go

@@ -0,0 +1,38 @@
+package test
+
+// Code generated by github.com/Khan/genqlient, DO NOT EDIT.
+
+import (
+	"time"
+
+	"github.com/Khan/genqlient/graphql"
+)
+
+type convertTimezoneResponse struct {
+	Convert time.Time `json:"convert"`
+}
+
+func convertTimezone(
+	client graphql.Client,
+	dt time.Time,
+	tz string,
+) (*convertTimezoneResponse, error) {
+	variables := map[string]interface{}{
+		"dt": dt,
+		"tz": tz,
+	}
+
+	var retval convertTimezoneResponse
+	err := client.MakeRequest(
+		nil,
+		"convertTimezone",
+		`
+query convertTimezone ($dt: DateTime!, $tz: String) {
+	convert(dt: $dt, tz: $tz)
+}
+`,
+		&retval,
+		variables,
+	)
+	return &retval, err
+}