Add option to un-flat the error object

Feature request:

Add option to allow to return an un-flat error object by adding an options argument for Validator.

/**
 * Default options
 * {
 *   flat: Boolean (Default: true)
 * }
 */
const validation = new Validator(data, rules, {flat: false});
const errors = validation.errors.all();
// > {foo: {bar: 1}}

const validation = new Validator(data, rules);
const errors = validation.errors.all();
// > {'foo.bar': 1}

Personally I would find this option useful when using with redux-form where the validation function must return an nested object structure as received.

Additionally the options argument could be extended with more features e.g. instance only rules (proposed #98 )

Could be easily added using flat.unflatten, rather then implementing your own unflatten method.

I can implement the changes if you agree with my proposal.

[MikeSuiter]

+1 for redux-form nested errors support.

This library will do the trick but having one less dependency would be nice.

unflatten(validator.errors.all())

https://github.com/hughsk/flat

[iamdtang]

How about we make options the 4th argument to ensure backwards compatibility. If options needs to be passed, the invocation would look like this:

const validation = new Validator(data, rules, {}, { flat: false });