Add -exec CLI argument to execute commands when micro starts (#4132)

This argument allows us to execute one or more commands immediately
after editor initialization.
This is useful in cases where you know you are going to want to open
two panes immediately, or for scripting purposes so you can start
with certain things set up, or if you want to execute certain plugin
actions at startup, etc.

Usage:
	micro -exec "...cmd..."
	micro -exec "cmd1" -exec "cmd2" -exec "cmd3" ...

(multiple commands are gathered into an array and executed in order)
Commands are executed after plugin initialization, so you can use
plugin commands as well.

Updated man page with information about the new argument.

assets/packaging/micro.1

@@ -65,6 +65,12 @@ Enable CPU profiling (writes profile info to ./micro.prof so it can be analyzed
 .RS 4
 Show the version number and information and exit
 .RE
+.PP
+.B \-exec
+.I command
+.RS 4
+Run a command after the editor initializes (may be repeated)
+.RE
 
 Micro's plugins can be managed at the command line with the following commands.
 .RS 4

cmd/micro/micro.go

@@ -29,6 +29,15 @@ import (
 	lua "github.com/yuin/gopher-lua"
 )
 
+// multiStringFlag is a flag.Value that accumulates multiple values.
+type multiStringFlag []string
+
+func (f *multiStringFlag) String() string { return "" }
+func (f *multiStringFlag) Set(v string) error {
+	*f = append(*f, v)
+	return nil
+}
+
 var (
 	// Command line flags
 	flagVersion   = flag.Bool("version", false, "Show the version number and information")
@@ -38,6 +47,7 @@ var (
 	flagProfile   = flag.Bool("profile", false, "Enable CPU profiling (writes profile info to ./micro.prof)")
 	flagPlugin    = flag.String("plugin", "", "Plugin command")
 	flagClean     = flag.Bool("clean", false, "Clean configuration directory")
+	flagExec      multiStringFlag
 	optionFlags   map[string]*string
 
 	sighup chan os.Signal
@@ -68,6 +78,8 @@ func InitFlags() {
 		fmt.Println("    \tso it can be analyzed later with \"go tool pprof micro.prof\")")
 		fmt.Println("-version")
 		fmt.Println("    \tShow the version number and information and exit")
+		fmt.Println("-exec command")
+		fmt.Println("    \tRun a command after the editor initializes (may be repeated)")
 
 		fmt.Print("\nMicro's plugins can be managed at the command line with the following commands.\n")
 		fmt.Println("-plugin install [PLUGIN]...")
@@ -95,6 +107,7 @@ func InitFlags() {
 	for k, v := range config.DefaultAllSettings() {
 		optionFlags[k] = flag.String(k, "", fmt.Sprintf("The %s option. Default value: '%v'.", k, v))
 	}
+	flag.Var(&flagExec, "exec", "Run a command after the editor initializes (may be repeated)")
 
 	flag.Parse()
 
@@ -478,6 +491,10 @@ func main() {
 		// time out after 10ms
 	}
 
+	for _, cmd := range flagExec {
+		action.MainTab().CurPane().HandleCommand(cmd)
+	}
+
 	for {
 		DoEvent()
 	}