Add support for phan socket

Phan is already available #652 and got support for the client as well #1561.
By default an TCP connection is established.

In case someone works on a lot of different projects, it might be more handy to use the supported socket, to create a different socket per project.

Support for that would be really nice. I hacked something locally which might not be a good implementation. I'll provide a PR and hope someone with more Vimscript experience can check the implementation.

From help of the client:
```bash
./vendor/bin/phan_client -h
Usage: ./vendor/bin/phan_client [options] -l file.php [ -l file2.php]
--daemonize-socket
Unix socket which a Phan daemon is listening for requests on.

--daemonize-tcp-port
TCP port which a Phan daemon is listening for JSON requests on, in daemon mode. (E.g. 'default', which is an alias for port 4846)
If no option is specified for the daemon's address, phan_client defaults to connecting on port 4846.
```