Added new optional argument to random for gifs on a subject

I've added an optional argument for getting random gifs (`Giphy.random`) so that random gifs can be found from a specific tag. I noticed it in Giphy's API but not in yours so I thought I'd try my hand and add it :smiley: 
I don't think we need to receive more than one argument here, optionally receiving a tag would be enough, making the interface simpler. What do you think?