Allow pre-validating against itself or another set of attributes

[ragulka]

This PR is a solution to #211. It allows pre-validating a set of attributes against itself or another set of attributes.

// Validate a hash of attributes against itself
// Pass in either `true` or an attribute hash as the 2nd parameter.
// Neither of the examples below will result in any errors.
var errors = model.preValidate({ password: 'value', passwordConfirmation: 'value' }, true);
var errors = model.preValidate({ password: 'value' }, { passwordConfirmation: 'value' });

Note: I did write the tests, but since npm install buster failed, I couldn't actually run them.

[chiefGui]

+1! @thedersen

[dottodot]

Any update on this?