Add skipPresence option.

[danhper]

Hi,

This is a PR to add a skipPresence options to validate.
This is useful to validate data on update.

For example, this would allow to write something like this.

var options = {};
if (request.method === 'PUT') {
  options.skipPresence = true;
}
validate(request.body, constraints, options);

Thanks.

[ansman]

This could be solved today like this:

var constraints = {
  foo: {
    presence: function(value, attributes, attribute, options, constraints) {
      return options.method !== 'PUT' ? {} : null
    }
  }
};

validate({}, constraints, {method: 'POST'});
// {"foo": ["Foo can't be blank"]}

validate({}, constraints, {method: 'PUT'});
// undefined

I understand this is not as nice as having a dedicated option for it but I don't want to add validator specific options unless there is a very good reason.

I'm in the processing of implementing a change that will pass the global options to a custom validator which means you could do something like this:

var presence = validate.validators.presence;
validate.validators.presence = function(value, options, key, attributes, globalOptions) {
  return globalOptions.method === 'PUT' ? null : presence.call(presence, arguments);
};

It should be ready within the hour.

[danhper]

This seems to be a more flexible alternative, thanks!
I am closing this PR, as 7f5355d solves the issue.