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 of 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.