rpcc: Inherit proxy configuration from environment

This is what the gorilla/websocket default dialer does already, so it
makes sense to use it by default.

Fixes #94 (or at least partially).

rpcc/conn.go

diff --git a/rpcc/conn.go b/rpcc/conn.go
index aa210df..3bcef1d 100644
--- a/rpcc/conn.go
+++ b/rpcc/conn.go
@@ -9,6 +9,7 @@
 	"io"
 	"log"
 	"net"
+	"net/http"
 	"sync"
 
 	"github.com/gorilla/websocket"
@@ -122,6 +123,7 @@
 	if netDial == nil {
 		netDial = func(ctx context.Context, addr string) (io.ReadWriteCloser, error) {
 			ws := &c.dialOpts.wsDialer
+			ws.Proxy = http.ProxyFromEnvironment
 
 			if ws.WriteBufferSize == 0 {
 				// Set the default size for use in writeLimiter.