support .xliff file extension in pomerge

`pomerge` 2.2.3 does not recognize .xliff files as XLIFF, only .xlf files.  Apple has [standardized on .xliff](https://developer.apple.com/library/content/documentation/MacOSX/Conceptual/BPInternational/LocalizingYourApp/LocalizingYourApp.html), and all OSes support more than 3-char file extensions for a very long time.