Possibility to set a max number of relations

[a-barbieri]

Expected behavior

It should be possible to explicit a max number of relations which can be 3, 4, 6 but even a single relation is important!

Actual behavior

Currently the user can add as many as he wants

Binda version: https://github.com/a-barbieri/binda/commit/b4b4ce8fc6ad0f16caf24f7c9b0ac18371819386

Rails version: 5.1.4