Added asDuration() function to yii\i18n\Formatter.php

Closes #5146 

With the weird outputs for certain locales, and the lack of other units than hours, minutes, and seconds in the output no matter how big the input is, I gotta say that I don't see any point in using the PHP intl extension. At least not for durations (haven't checked the other functions yet).

I'm not too familiar with Yii2 yet, so I'm not sure if there are any classes or methods that could be used to improve the code or make it more customizable, but it works and might be something we can use as base and then improve upon.