Support jstype overrides

[luk3thomas]

This PR should address the issues mentioned in #10.

If I add the [jstype=JS_STRING] option to a message then the generated stubs will return a string for the 64-bit integer value. The generated typescript definition does not handle this.

I didn't see any tests, but I did give my changes a go and I'm getting the results I expected.

event.proto

message Event {
  fixed64 id = 1 [jstype = JS_STRING];
}

Generated interface

 export class Event extends jspb.Message {
-    getId(): number;
-    setId(value: number): void;
+    getId(): string;
+    setId(value: string): void;

     getGeneratedAt(): number;
     setGeneratedAt(value: number): void;
@@ -272,7 +272,7 @@ export class Event extends jspb.Message {

 export namespace Event {
     export type AsObject = {
-        id: number,
+        id: string,
         generatedAt: number,
         receivedAt: number,
         sourceId: number,

Fixes #10

[agreatfool]

That's awesome. Thanks for the pr.

I'm a bit busy these days. Please give me several days to merge it. Thanks.