Add flag to list directives.

main.go

@@ -14,6 +14,7 @@ import (
 
 	"github.com/mholt/caddy/caddy"
 	"github.com/mholt/caddy/caddy/https"
+	"github.com/mholt/caddy/caddy/parse"
 	"github.com/xenolf/lego/acme"
 	"gopkg.in/natefinch/lumberjack.v2"
 )
@@ -36,6 +37,7 @@ func init() {
 	flag.StringVar(&revoke, "revoke", "", "Hostname for which to revoke the certificate")
 	flag.StringVar(&caddy.Root, "root", caddy.DefaultRoot, "Root path to default site")
 	flag.BoolVar(&version, "version", false, "Show version")
+	flag.BoolVar(&directives, "directives", false, "List supported directives")
 }
 
 func main() {
@@ -77,6 +79,12 @@ func main() {
 		}
 		os.Exit(0)
 	}
+	if directives {
+		for d := range parse.ValidDirectives {
+			fmt.Println(d)
+		}
+		os.Exit(0)
+	}
 
 	// Set CPU cap
 	err := setCPU(cpu)
@@ -217,6 +225,7 @@ var (
 	logfile    string
 	revoke     string
 	version    bool
+	directives bool
 )
 
 // Build information obtained with the help of -ldflags