Switch to gometalinter

Only small amount of required checks is enabled,
plan is to enable more linters as issues are fixed in the code.

cmd/snapshot_diff.go

@@ -2,6 +2,7 @@ package cmd
 
 import (
 	"fmt"
+
 	"github.com/smira/commander"
 	"github.com/smira/flag"
 )