Add ability to set HTTP Client Request checkStatus Function

Thanks for the module! It's always nice to have places around hackage that have been all prettied up with lenses! :)

I hit a bit in a program that I was writing where I needed to get to the body of a response that was not a 2XX response. I couldn't find a way in wreq to set the checkStatus function of the underlaying http client so I added it into the options data type. 

The test case that I wrote passes and I have also tested the updated version in the application that I was running and it works just swimmingly, but I'm happy to take comments and make changes as you need or want me to. 
Typo: "returs" -> "returns".
Sentence needs a trailing ".".