add function to manually setup proxy. Useful on iOS where getenv doesn't return proxy settings

On iOS getenv("http_proxy") doesn't return proxy settings so you have to use some iOS SDK (CFNetwork) calls to get proxy. I have added this function to be able to set proxy manually after I got it from system. I have considered also to add proxy setting as a parameter to libwebsocket_create_context but I thought it would be to intrusive and not that commonly used. Please  check if it suits the overall style of library since maybe I have missed something. If the code is not OK, please add the functionality to set proxy as you think would be best.