Add operator to fetch cookies by their key

[Callen44]

The main idea of this PR is to add the ability to fetch cookies by their individual key from a cpr::Cookies object. calling the [] operator on a string storing the cookie name would return the actual cookie with that name, if not, it throws std::out_of_range. Looping through the list of cookies produces annoying boilerplate code, and this would fix that. I'm a new C++ programmer; worse yet, my introduction to C++ was with version 98, so I won't be offended by quite possibly a lot of feedback.

I already have these concerns with my own PR, however.

1. The idea of a Cookies object is to be vector-like, and this code intentionally more map-like; I'm not sure if the very concept is out-of-place or not.
2. Looping through cookies like this could become very inefficient for large cookie lists (I'm not sure if that kind of speed is a priority in cpr or not).
3. There's always the problem of a given cookie not being possible to find, I'd love some help on whether throwing std::out_of_range is the right way to handle that.