Add "localizednumber" and "localizedcurrency" filters

Hello,

This is a new version of PR #55 proposed by @Garfield-fr:

* Rebased on the `master` branch
* Made some changes according to comments made by @stof: remove unneeded arguments, checks on provided values
* Added documentation

As for the name of these filters, I would go for `localized_date`, `localized_number` and `localized_currency` but as `localizeddate` already exists, I preferred to keep the (weird) naming convention to keep backward compatibility.

Naming conventions changes could be made in an other PR, for a future version of Twig.

I didn't add tests as there is currently no test for any filter (apart from grammar checks).

I can squash all commits into one if needed.
I rather not create a new object for every call.
You can use a static variable (not property) and check if style and locale differ.

Note. Quick example, not tested ;) 

```
static $formatter $currentStyle;

$locale = $locale :? Locale::getDefault();

if (!$formatter || $formatter->getLocale() != $locale || $currentStyle != $style) {
    $currentStyle = $style;
    $formatter = NumberFormatter::create($locale, $styleValues[$style]);
}
```