Add support for :array option in Pastore::Params

[serioja90]

It would be nice to have the possibility to validate arrays of specific type (e.g. param :status, type: :string, array: true, in: ['draft', 'active', 'inactive', 'expired']).

The option :array will indicate that the param is an array, so should be treated as array.

If a single value is passed, like status=draft, it should be automatically converted to an array of strings and then validated.

For array inputs the :in option should check if all the values in array respect the inclusion requirement.