Add gomodtidy to post update (#21)

Author: firecow

renovate.json

@@ -1,3 +1,4 @@
 {
-	"extends": ["config:best-practices", "config:recommended", ":gomod", ":maintainLockFilesMonthly", ":disableDependencyDashboard"]
+	"extends": ["config:best-practices", "config:recommended", ":gomod", ":maintainLockFilesMonthly", ":disableDependencyDashboard"],
+	"postUpdateOptions": ["gomodTidy"]
 }