Add unix domain socket support

[na-trium-144]

I wanted to use unix domain socket since performance is important in my project, so I implemented based on ipkn/crow#379.
Instead of duplicating App and Server classes, I created wrapper class of acceptor so the existing Server class can be used for unix socket connection.

By specifying socket path with app.unix_path() (instead of app.port(port_num)) the server will create and use that socket instead of listening a tcp socket. (I added example as example/example_unix_socket.cpp)
Although it is named unix socket, this also works on windows.

If there is any problem please let me know.
Especially I am not sure if there is a better name than unix_path().

[mrozigor]

Maybe rename it if it also works on Windows? Maybe something like external_socket_path or something like this?

[na-trium-144]

Actually it is named "unix socket" (or AF_UNIX) on Windows: https://devblogs.microsoft.com/commandline/af_unix-comes-to-windows/
But Asio's documentation says it is also known as local sockets, so maybe local_socket_path is ok if "unix" is confusing.
https://think-async.com/Asio/boost_asio_1_18_0/doc/html/boost_asio/overview/posix/local.html