Replace flag library to be able to use EnvVars

glide.lock

 hash: 09dcc55682d2ed07028b0682a29000bd39d1c225a2cd313e9b9049e72b990cd2
-updated: 2018-06-07T14:53:28.640281981+02:00
+updated: 2018-06-07T15:26:42.502950667+02:00
 imports:
 - name: github.com/beorn7/perks
   version: 4c0e84591b9aa9e6dcfdf3e020114cd81f89d5f9
@@ -13,6 +13,8 @@ imports:
   version: c12348ce28de40eed0136aa2b644d0ee0650e56c
   subpackages:
   - pbutil
+- name: github.com/namsral/flag
+  version: 71ceffbeb0ba60fccc853971bb3ed4d7d90bfd04
 - name: github.com/prometheus/client_golang
   version: 967789050ba94deca04a5e84cce8ad472ce313c1
   subpackages:

main.go

 package main
 
 import (
-	"flag"
+	"github.com/namsral/flag"
 	"net/http"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"fmt"