add option to support text format (#54)

* augments grpcurl package API in order to handle multiple formats
* deprecates old signature for InvokeRpc
* add command-line flag to use protobuf text format instead of JSON
* use AnyResolver when marshaling to/from JSON

testing/example.proto

@@ -3,9 +3,11 @@ syntax = "proto3";
 import "google/protobuf/descriptor.proto";
 import "google/protobuf/empty.proto";
 import "google/protobuf/timestamp.proto";
+import "testing/example2.proto";
 
 message TestRequest {
 	repeated string file_names = 1;
+	repeated Extension extensions = 2;
 }
 
 message TestResponse {

testing/example2.proto

@@ -0,0 +1,8 @@
+syntax = "proto3";
+
+import "google/protobuf/any.proto";
+
+message Extension {
+    uint64 id = 1;
+    google.protobuf.Any data = 2;
+}