'goby' tool additions

[tsaubergine]

• Refactor 'goby' tool code for future expansion
• Add ability to read config from environmental variable with (goby.field).cfg.env extension
• Add 'goby protobuf show' command. Fixes #302

export GOBY_TOOL_LOAD_SHARED_LIBRARY=/home/toby/opensource/goby3-examples/build/lib/libgoby3_example_messages.so

> goby protobuf show protobuf.NavigationReport
============== protobuf.NavigationReport============== 
message NavigationReport {
  enum VehicleClass {
    AUV = 1;
    USV = 2;
    SHIP = 3;
  }
  required double x = 1;
  required double y = 2;
  required double z = 3;
  optional .protobuf.NavigationReport.VehicleClass veh_class = 4;
  optional bool battery_ok = 5;
}