Add command line option to tell protobuf compiler to generate multiple java files

[drmaas]

This would be added to the generated .proto file at the top, below package:

option java_multiple_files = true;

It looks like by adding this:

x-global-options:
  java_multiple_files: true

to the spec, the schema won't compile because it expects the option value to be a string. Some protobuf options, like java_multiple_files, are boolean.