Added a function to enable SSL using a certificate chain

[The-EDev]

pointed out and implemented by @jeanbiber, Here's a modified version of ssl_file that allows a certificate chain:

    self_t& ssl_chainfile(const std::string& crt_filename, const std::string& key_filename)
    {
        ssl_used_ = true;
        ssl_context_.set_verify_mode(boost::asio::ssl::verify_peer);
        ssl_context_.set_verify_mode(boost::asio::ssl::verify_client_once);
        ssl_context_.use_certificate_chain_file(crt_filename);
        ssl_context_.use_private_key_file(key_filename, ssl_context_t::pem);
        ssl_context_.set_options(
                boost::asio::ssl::context::default_workarounds
                      | boost::asio::ssl::context::no_sslv2
                      | boost::asio::ssl::context::no_sslv3
                );
        return *this;
    }

It might be useful to implement this in Crow.