add ability to specify .zeus.sock path

[barooo]

since you can specify the socket path to zeus via an environment variable, this lets us override the default (project_root/.zeus.sock) via a variable (in my specific case, "/tmp/zeus.sock").

[dgutov]

Why not read that variable's value with getenv?

[barooo]

Cleaned up the commits so there's only one, and took the suggestion to use getenv.

[dgutov]

Thanks!