Add godoc make target
Add godoc make target

This target launches the godoc http server on port 6060 for easy
browsing of the Pythia (and Go) documentation.

pythia-project/pythia,pythia-project/pythia,pythia-project/pythia-core,pythia-project/pythia,pythia-project/pythia-core,pythia-project/pythia,pythia-project/pythia-core,pythia-project/pythia-core,pythia-project/pythia-core,pythia-project/pythia,pythia-project/pythia-core,pythia-project/pythia