Cpp implementation for mg_get_cookie().

Hi I am Hariprasad Kamath

I was using mongoose since tw years on windows and qnx. Combined with my own c++ wrapper.

I am interested to contribute to this project.

Here's my first pull request:

C++ implementation for mg_get_cookie()
The user can pass standard strings for cookie name get back the corresponding value.
The method returns the size of the cookie string read.

Let me know if any changes/improvement required.