Abort if pending migrations + task chaining

This provides tasks to check for pending migrations like:

```
rake db:abort_if_pending_migrations:before
rake db:abort_if_pending_migrations:during
rake db:abort_if_pending_migrations:after
```

It also enables task chaining, like:

```
rake db:abort_if_pending_migrations:before db:abort_if_pending_migrations:during

rake db:migrate:list:before db:migrate:list:after
```