Add --exit-with-highest-exit-code which exits hivemind with the highest exit code of all processes

This allows for process failures to bubble up to the shell.

Example:

```shellsession
$ hivemind --exit-with-highest-exit-code - <<PROCFILE
job1: echo "job1 is running"; sleep 0.5; echo "Failing"; exit 13
job2: echo "job2 is running"; sleep 1; echo "Will never complete"; exit 11
PROCFILE
job2 | Running...
job1 | Running...
job2 | job2 is running
job1 | job1 is running
job1 | Failing
job1 | exit status 13
job2 | Interrupting...
job2 | signal: interrupt
At least one process failed
$ echo "Hivemind exited with: $?"
Hivemind exited with: 13
```

Author: danieroux

hivemind.go

@@ -21,6 +21,7 @@ type hivemindConfig struct {
 	Timeout            int
 	NoPrefix           bool
 	PrintTimestamps    bool
+	ExitWithHighest    bool
 }
 
 type hivemind struct {
@@ -97,7 +98,7 @@ func (h *hivemind) waitForExit() {
 	}
 }
 
-func (h *hivemind) Run() {
+func (h *hivemind) Run() int {
 	fmt.Printf("\033]0;%s | hivemind\007", h.title)
 
 	h.done = make(chan bool, len(h.procs))
@@ -112,4 +113,15 @@ func (h *hivemind) Run() {
 	go h.waitForExit()
 
 	h.procWg.Wait()
+
+	exitCode := 0
+
+	for _, proc := range h.procs {
+		code := proc.ProcessState.ExitCode()
+		if code > exitCode {
+			exitCode = code
+		}
+	}
+
+	return exitCode
 }

main.go

@@ -30,14 +30,15 @@ func main() {
 	app.HideHelp = true
 
 	app.Flags = []cli.Flag{
-		cli.StringFlag{Name: "title, w", EnvVar: "HIVEMIND_TITLE", Usage: "Specify a title of the application", Destination: &conf.Title},
-		cli.StringFlag{Name: "processes, l", EnvVar: "HIVEMIND_PROCESSES", Usage: "Specify process names to launch. Divide names with comma", Destination: &conf.ProcNames},
+		cli.StringFlag{Name: "title, w", EnvVar: "HIVEMIND_TITLE", Usage: "specify a title of the application", Destination: &conf.Title},
+		cli.StringFlag{Name: "processes, l", EnvVar: "HIVEMIND_PROCESSES", Usage: "specify process names to launch. Divide names with comma", Destination: &conf.ProcNames},
 		cli.IntFlag{Name: "port, p", EnvVar: "HIVEMIND_PORT,PORT", Usage: "specify a port to use as the base", Value: 5000, Destination: &conf.PortBase},
 		cli.IntFlag{Name: "port-step, P", EnvVar: "HIVEMIND_PORT_STEP", Usage: "specify a step to increase port number", Value: 100, Destination: &conf.PortStep},
 		cli.StringFlag{Name: "root, d", EnvVar: "HIVEMIND_ROOT", Usage: "specify a working directory of application. Default: directory containing the Procfile", Destination: &conf.Root},
 		cli.IntFlag{Name: "timeout, t", EnvVar: "HIVEMIND_TIMEOUT", Usage: "specify the amount of time (in seconds) processes have to shut down gracefully before being brutally killed", Value: 5, Destination: &conf.Timeout},
 		cli.BoolFlag{Name: "no-prefix", EnvVar: "HIVEMIND_NO_PREFIX", Usage: "process names will not be printed if the flag is specified", Destination: &conf.NoPrefix},
 		cli.BoolFlag{Name: "print-timestamps, T", EnvVar: "HIVEMIND_PRINT_TIMESTAMPS", Usage: "timestamps will be printed if the flag is specified", Destination: &conf.PrintTimestamps},
+		cli.BoolFlag{Name: "exit-with-highest-exit-code, e", EnvVar: "HIVEMIND_EXIT_WITH_HIGHEST_EXIT_CODE", Usage: "exit hivemind with highest exit code from all processes", Destination: &conf.ExitWithHighest},
 	}
 
 	app.Action = func(c *cli.Context) error {
@@ -65,7 +66,10 @@ func main() {
 		conf.Root, err = filepath.Abs(conf.Root)
 		fatalOnErr(err)
 
-		newHivemind(conf).Run()
+		exitCode := newHivemind(conf).Run()
+		if exitCode > 0 && conf.ExitWithHighest {
+			return cli.NewExitError("At least one process failed", exitCode)
+		}
 
 		return nil
 	}