Allow alternative means of hiding/showing error message.

Hi.

Currently, when using either default validation messages or explict (in-template) ones,
hiding and showing of them is done via `style="display: none;"` attribute:

``` html

```

or

``` html

```

which makes styling and especially animating the error message a pain.

Perhaps configuration option **instead** :

``` javascript
errorMessageVisibleClass: 'is-hidden'
```

so that, when **it is supposed to be hidden**, it would look like this:

``` html

```

would allow more flexibility?

EDIT: cleaned up hurried explanation.

Start by locating the validation-message rendering and configuration code for both default validation messages and the validationMessage binding. Check how hidden messages currently receive the inline display style and whether existing tests cover that behavior. Done means an option such as errorMessageVisibleClass can control the hidden state without relying on an inline style, with coverage for both message forms.

Written by the indexing model from the issue text.