Support the jstype field option

[timostamm]

This PR adds support for the jstype field option, which can be used to generate 64-bit integral fields as String instead of BigInt.

For example:

int64 my_field = 1 [jstype = JS_STRING];

Will generate a field typed as string:

/**
 * @generated from field: int64 field = 1 [jstype = JS_STRING];
 */
myField: string;

Closes #556.