Add -c flag to specify list of config files

- Environment variable also accepts comma-separated list
  - Overwrites cli arguments

main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"flag"
 	"fmt"
 	"log"
 	"net/http"
@@ -15,7 +16,22 @@ import (
 	"gitlab.hamburg.ccc.de/ccchh/spaceapid/util"
 )
 
+var (
+	flagHelp bool
+)
+
+func init() {
+	flag.BoolVar(&flagHelp, "h", false, "Print help output")
+}
+
 func main() {
+	flag.Parse()
+
+	if flagHelp {
+		flag.PrintDefaults()
+		os.Exit(0)
+	}
+
 	// Get spaceapid configuration
 	conf := config.ParseConfiguration()