Support for Unix-domain Sockets

[grayside]

In reviewing the documentation and code I cannot see how Unix-domain sockets (https://www.postgresql.org/docs/9.5/libpq-connect.html#AEN43017) are supported.

#19 implies it may be supported.

Is it currently supported? If not, would you consider adding support? How can I help?

[adoakley]

I added support for unix sockets from nginx in my fork, commit 3f6bbc0. It would be better if it was supported for the other sockets libraries too, but it's unlikely that I'll every get around to implementing that.

Feel free to merge it if you think it's good enough.