feature: add maximum data size to Parse signature

[dadleyy]

currently when parsing multipart/form-data requests, the size of data is locked in at 2048. For image uploads this might be a little low; is it possible to add the maximum size to the functions signature? Or perhaps a ParseMax(*http.Request, uint) function?

[albrow]

@dadleyy this is a good idea. I will not have time to do it anytime soon but I would be more than happy to review a PR if you are interested in tacking a stab at it.