hdcscy

New function "stream_file" about "append" flag

2

I have a simple demo here on windows: ``` void async_append(const std::string& data, const std::function& handler) { try { boost::asio::stream_file file( m_ctx, *m_path, boost::asio::stream_file::append); if (file.is_open()) { file.async_write_some( boost::asio::const_buffers_1(data.data(), data.size()),...