add the --verify option

This is the second attempt at #535 - this time targeting the gulp 4 branch. I think I've tackled all the comments left in the original PR (#668) 
Yeh, I didn't feel like introduce another dependency just for this one HTTP request as it felt for me that `request` is just too much for this simple use case and I didn't know any other package that would be small & do the job. 

@phated any other suggestions than `superagent` (BTW: what is wrong with it? I don't know it at all, just curious).