Add an option to print 64-bit integers without quotes

[svanharmelen]

I appreciate the fact that this is done because it is described as such in the ProtoJSON specification, but it prevents using the generated JSON in (most) other systems as it's not following RFC8259 which does not specify nor require 64-bit integers to be quoted.

By adding this option people still get the ProtoJSON output by default, but have the option to opt-out from the quoted 64-bit integers in order for the output to be more inline with RFC8259.

Hope this is something you are willing to merge as it will allow us to use the protobuf-json-mapping crate for our use cases. Thanks!