Export used CommandExecutor for testing DI

[lneves12]

Export the command executor, so we can mock all the executed commands for testing.

Explanation: https://npf.io/2015/06/testing-exec-command/

func fakeExecCommand(command string, args ...string) *exec.Cmd {
	cs := []string{"-test.run=TestHelperProcess", "--", command}
	argWithEnv := append(args, "GO_TEST_PROCESS")
	cs = append(cs, argWithEnv...)
	cmd := exec.Command(os.Args[0], cs...)
	return cmd
}

const commandMock = "OutputMock"

func TestHelperProcess(t *testing.T) {
	if os.Args[len(os.Args)-1] != "GO_TEST_PROCESS" {
		return
	}

	defer os.Exit(0)
	fmt.Fprintf(os.Stdout, "%s", dockerRunResult)
}

It("new test", func() {
	cmd.ComandExecuter = fakeExecCommand
	defer func() { cmd.ComandExecuter = exec.Command }()

        ... testing here.....
})

Issue: #45

[coveralls]

Coverage remained the same at 100.0% when pulling c6350db on lneves12:export_os_exec into a151197 on go-cmd:master.

[daniel-nichter]

See #45 for explanation. Thank you, though, for the PR.