T/92: Provide more secure translations upload / download

[ma2ciek]

This PR fixes #92 issue.

It adds token verification to provide more secure translations upload / download

Now to upload / download:

1. Generate your own token at https://www.transifex.com/user/settings/api/
2. Run gulp translations:download / gulp translations:upload

~/w/ckeditor5> gulp translations:download
[17:00:53] Using gulpfile ~/workspace/ckeditor5/gulpfile.js
[17:00:53] Starting 'translations:download'...
? Provide Transifex token (it should be generated here: https://www.transifex.com/user/settings/api/) *******************

It changes ckeditor/ckeditor5#387 process.