title: Automatic db router for migrations in django
body: <p>I am building a Django powered site and I want to have separate databases for some of the apps, I build a flexible router that routes each app to predefined database, this works fine.
  The problem is that when I am migrating my models I have to set the <code>--database</code> parameter every time and I find this annoying and redundant. Also many times I flooded my default database with tables from migrated app (by forgetting to add <code>--database</code>).
  I experimented with <code>allow_migrate(...)</code> function in my router but all I could achieve is safety mechanism that will not run the migration if I forget to specify the database.
  My question is: is there a way to set up a automatic database selection for models migrations in Django?
  My approach might be wrong, it surprises me that no one seems to have done that before.</p>