isValid: accept forceValidation parameter with fields, not just alone

`this.model.isValid(true)` is a great way to check to see if the entire model is valid and trigger validate() in one line of code. I imagine this was implemented because `_.isUndefined(this.model.validate())` is hideous.

This pull request lets you do the same thing, but only for a selected field or group of fields. `this.model.isValid('name', true)` or `this.model.isValid(['name', 'age'], true)` will check to see if the specified fields are valid, and trigger validate() on the specified model attributes in one line of code.

I needed this because I have a view that is more or less a "wizard". When the user clicks the "next" button, I have to validate that all the fields on that page of the form are valid, and show errors, without validating any fields on future pages of the wizard (and are therefore undefined).

Thanks for an awesome library. Please let me know if there's any way I can improve this request.