Add flag for dumping a specific field.

Only ID field is supported right now. Others can be added very easily.
Initial implementation.

Only runs one pass through the FS Persons.

Run as:
go run src/github.com/rootsdev/fsbff/finddescendants/finddescendants.go
-d descendants.txt -p persons.proto -o out.txt

If satisfied with output, replace descendants.txt with out.txt and run
again. Repeat about a dozen times.

Note: Has not been tested on more than one proto file.