Add 'print_message_fields_in_index_order' option to text formatter

[bouk]

Just like the C++ printer. I'll let the comment explain what it does:

// If print_message_fields_in_index_order is true, fields of a proto message
// will be printed using the order defined in source code instead of the
// field number, extensions will be printed at the end of the message
// and their relative order is determined by the extension number.
// By default, use the field number order.

[andrewhickman]

Do you have a particular usecase for this option? I'd like to avoid supporting too many formatting options if possible

[bouk]

I'm working on an editor for protobuf files and I want to have the order of the fields match the order in the proto definition, purely for aesthetic reasons.

[andrewhickman]

I'd accept a PR to implement this, so long as it doesn't require any extra work when building the DescriptorPool to track the ordering. Hopefully it can be implemented using the original descriptor_proto