Allow chef-zero to listen on Unix domain socket.

Puma has support for unix domain sockets.  This commit adds a -s
option to chef-zero that allows the user to specify a path rather than
an IP and port:

    > chef-zero -s /tmp/chef-zero.sock

Clients can then connect to this socket:

    > telnet -u /tmp/chef-server.sock
    Trying /tmp/chef-server.sock...
    Connected to (null).
    Escape character is '^]'.
    GET /nodes HTTP/1.1

    HTTP/1.1 200 OK
    Content-Type: application/json
    Content-Length: 3
    {
    }

1 files changed, 4 insertions, 0 deletions

diff --git a/bin/chef-zero b/bin/chef-zero
index b6b8a97..2d2f492 100755
--- a/bin/chef-zero
+++ b/bin/chef-zero
@@ -20,6 +20,10 @@ OptionParser.new do |opts|
     options[:port] = value
   end
 
+  opts.on("-s", "--socket PATH", String, "Unix socket path to listen on") do |value|
+    options[:sock] = value
+  end
+
   opts.on("--[no-]generate-keys", "Whether to generate actual keys or fake it (faster).  Default: false.") do |value|
     options[:generate_real_keys] = value
   end