Split cmd for readability sake

[bpineau]

Try to ease discovery/readability by splitting cmd (cobra/viper
setup) in various files. The proeminent cmd/execute.go should
hopefuly make the flow clear:

main() -> execute -> run.Run() -> {http, recorder, observer, events, git}

[coveralls]

Coverage increased (+0.01%) to 91.195% when pulling d787f20 on split_cmd into 6fa5415 on master.