Add the ability to create custom validation on fields

lib/validatable.js

@@ -10,6 +10,16 @@ Validatable.validatesNumericalityOf = getConfigurator('numericality');
 Validatable.validatesInclusionOf = getConfigurator('inclusion');
 Validatable.validatesExclusionOf = getConfigurator('exclusion');
 Validatable.validatesFormatOf = getConfigurator('format');
+Validatable.validate = function(){
+    args = [].slice.call(arguments);
+    var valFn = function(){}; //noop
+    if (typeof args[args.length - 1] === 'function'){
+        valFn = args.pop();
+    }
+    wrapperFn = function(attr, conf, err){ return valFn.call( this, err) };
+    configure(this, wrapperFn, args)
+
+}
 
 // implementation of validators
 var validators = {
@@ -118,7 +128,8 @@ function validationFailed(inst, v) {
     if (skipValidation(inst, conf, 'unless')) return false;
 
     var fail = false;
-    validators[conf.validation].call(inst, attr, conf, function onerror(kind) {
+    var validator = typeof conf.validation === "function" ? conf.validation : validators[conf.validation];
+    validator.call(inst, attr, conf, function onerror(kind) {
         var message;
         if (conf.message) {
             message = conf.message;

test/validations_test.coffee

@@ -215,3 +215,16 @@ it 'should validate format', (test) ->
 
     test.done()
 
+it 'should validate a field using a custom validator', (test)->
+    
+    User.validate 'email', (err)-> err("crash") if @email.length is 0
+
+    user = new User validAttributes
+    test.ok user.isValid()
+
+    user = new User validAttributes
+    user.email = ""
+    test.ok not user.isValid()
+
+    test.done()
+