Callback last argument

[NaridaL]

for deserialize and update

fixes #77

I'm not loving this, tbh... you always need to pass the custom arguments now. If you forget them, you won't get a compiler error, as callback is optional, and customArg is typed as any.

Converting to promises completely, as @alexggordon suggested in #77 is problematic too, as Promises always return asynchronously, and the fact that serializr returns synchronously if there are no async dependencies is great.

util.promisify offers a mechanism for a custom promisified function, by defining deserialize[util.promisify.custom] = function..., this would be a non-breaking change which would probably be better. TypeScript also correctly types the custom promisied function.

[coveralls]

Coverage increased (+0.02%) to 88.889% when pulling a1afa26 on callback-last-argument into 3738fe1 on master.