Adding support to setting virtuals

Sorry, a bit late, but I just added this in. Virtuals can now also have a setter function:

    var User = Bookshelf.Model.extend({
      tableName: 'users',
      outputVirtuals: true,
      virtuals: {
        fullName: function() {
          return this.get('firstName') + ' ' + this.get('lastName');
        },
        fullNameUpperCase: {
          get: function() {
            return this.get('firstName') + ' ' + this.get('lastName');
          },

          set: function(value) {
            value = value.split(' ');
            this.set('firstName', value[0].toUpperCase());
            this.set('lastName', value[1].toUpperCase());
          },
        }
      },
    });