Ability to generate OASIS XLIFF 1.2 languages files from tbg_cli added.

As discussed, ```tbg_cli``` ```extract_strings``` task is now able to output ```.xlf``` files complying to [OASIS XLIFF 1.2](http://docs.oasis-open.org/xliff/xliff-core/xliff-core.html) standard :earth_americas: 
```--format``` parameter has been added. It can be either ```php``` or ```xliff```

I have tested _Transifex_ import and it works like a charm :heart: 

Enjoy :wink: