Bug: `MalformedLocatorException` thrown for `id`s that don't start with a character (HTML 5)

These both result in an `MalformedLocatorException`:
```php
$I->seeElement(['id' =>'123']);
$I->seeElement(['css' =>'div#123']);
```
The reason is probably that up to HTML 4, `id`s were required to start with a character (a-z). However, in HTML 5, `id`s can begin with virtually anything: https://html.spec.whatwg.org/multipage/dom.html#the-id-attribute