Adapt seeder to an abstract class

[jorgebodega]

To fix #119 we need to find a way to avoid the problem of export multiple elements but not checking if those are seeder.

A possible approach is to define Seeders as an abstract class instead of an interface. That will allow us to use instanceof to infer what could be used as a seeder. Similarly to new seeder of MikroORM.

Or we can discuss about allowing only default exports

[micalevisk]

+1 to class MySeeder extends Seeder { ... } approach.

[jorgebodega]

Closed by #191