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. 
Please make sure that Haddock has hyperlinks to type and value names, so `'Nothing'` here, and `'Network.HTTP.Client.Request'` below, and `'StatusException'` also.