added support for js_type using bigint

[UlysseM]

Writing this as a proof of concept to get some feedback. (my take on #44)

This approach is intended to be backward compatible with existing system. When dealing with int64* family, it checks whether the option jstype is set. If set to JS_STRING, it returns a string and parses a string. If set to JS_NUMBER, it returns a BigInt and parses a BigInt.

API wise, if we don't want to introduce BigInt in the index.js (to maintain compatibility with older browser without the need to ship polyfills), we could:

• Move the newly added logic from index.js to a separate file (jstype.js), and have that file inject the functions to Pbf. The user would need to import 'pbf' and 'pbf/jstype' for this to work.
• Alternatively, we could write the code that inject the functions to Pbf in the generated file, if it uses the feature.

Let me know if you're ok with this approach / feature, so that I can polish it / write tests.

[UlysseM]

A bit too complex after further thoughts. I added support for js_type at compile time without the needs of bigint.

It doesn't solve number in the order of 2^53, but support for it could be added later.