Option to validate the error always on non-required fields from custom validation

Added a new property to the fields object so that you could always validate the error thrown the custom validation by non-required fields.

Added a test also to validate it: 
``` javascript
  "field : validateErrorsAlways : custom not required with error" : function() {
    var request = {
      body: {
        field0: "",
      }
    };
    var helpers = {
      customHelperWithError: function(data){
        throw new Error('fail');
      }
    };
    form(
      field("field0").custom(helpers.customHelperWithError).validateErrorsAlways()
    )(request, {});
    assert.equal(request.form.isValid, false);
    assert.equal(request.form.errors.length, 1);
  }
```