New validation rule sometimes_without

[troccoli]

Write a new validation rule called sometimes_without which will accept a list of fields, .e.g 'season' => 'sometimes_without:competition,division'.

The rule will fail if the field is present (i.e. can be empty) and any of the fields in the rules is present too.

So in the example, the rule fails if season is present and either competition or division, or both, are present too.

Blocking #141

[troccoli]

There is now a Laravel rule: prohibits