Date stringify

This fixes it so that:

```jade
foo(data-epoc=new Date(0))
```

becomes

```html
<foo data-epoc="1970-01-01T00:00:00.000Z"></foo>
```

It also adds warnings for:

```jade
foo(value=new Date(0))
```

which currently becomes:

```html
<foo value="Thu Jan 01 1970 00:00:00 GMT+0000 (BST)"></foo>
```

but in jade 2.0.0, will (unless people object) become:

```html
<foo value="1970-01-01T00:00:00.000Z"></foo>
```

See #1062 for the original issue.