Fixes #996: Allow connecting to Postgres via a Unix socket

I've tested it locally, and it seems to work fine both for connecting to a postgres server or a postgres unix socket. I hesitated a bit about asking the port when using the socket, but it seems that typeorm uses this to determine the socket's file name, so I let it be mandatory. Waiting for feedback from postgres unix socket users to know whether it's common practice or not :-)

(We could argue that defining the port in general when using the postgres server is overkill, since we could use the default when it's not set.)