feat: add support set http-proxy-redir-url in nginx proxy.

```
location /web/ {
    proxy_set_header HttpProxyRedir http://$server_name;
    proxy_pass http://127.0.0.1:5913;
}
```

Signed-off-by: Jianhui Zhao <zhaojh329@gmail.com>

README.md

@@ -132,6 +132,33 @@ server {
 }
 ```
 
+The parameter 'http-proxy-redir-url' in rttys.conf can also be configured
+by setting a new HTTP header 'HttpProxyRedir' in nginx.
+
+```
+server {
+    listen 80;
+
+    server_name rtty.your-server.com;
+
+    location /connect/ {
+        proxy_http_version 1.1;
+        proxy_set_header Upgrade $http_upgrade;
+        proxy_set_header Connection "Upgrade";
+        proxy_pass http://127.0.0.1:5913;
+    }
+
+    location /web/ {
+        proxy_set_header HttpProxyRedir http://$server_name;
+        proxy_pass http://127.0.0.1:5913;
+    }
+
+    location / {
+        proxy_pass http://127.0.0.1:5913;
+    }
+}
+```
+
 ## Docker
 ### Simple run
 

README_ZH.md

@@ -130,6 +130,46 @@ server {
 }
 ```
 
+在 rttys.conf 中的参数 http-proxy-redir-url 也可以通过在 nginx 中设置一个新
+的 HTTP header HttpProxyRedir 来配置.
+
+```
+server {
+    listen 80;
+
+    server_name rtty.your-server.com;
+
+    location /connect/ {
+        proxy_http_version 1.1;
+        proxy_set_header Upgrade $http_upgrade;
+        proxy_set_header Connection "Upgrade";
+        proxy_pass http://127.0.0.1:5913;
+    }
+
+    location /web/ {
+        proxy_set_header HttpProxyRedir http://$server_name;
+        proxy_pass http://127.0.0.1:5913;
+    }
+
+    location / {
+        proxy_pass http://127.0.0.1:5913;
+    }
+}
+
+server {
+    listen 80;
+
+    server_name web.your-server.com;
+
+    location / {
+        proxy_http_version 1.1;
+        proxy_set_header Upgrade $http_upgrade;
+        proxy_set_header Connection "Upgrade";
+        proxy_pass http://127.0.0.1:5914;
+    }
+}
+```
+
 ## Docker
 
     sudo docker run -it -p 5912:5912 -p 5913:5913 -p 5914:5914 zhaojh329/rttys:latest \

http.go

@@ -307,7 +307,10 @@ func httpProxyRedirect(br *broker, c *gin.Context) {
 		return
 	}
 
-	location := cfg.HttpProxyRedirURL
+	location := c.Request.Header.Get("HttpProxyRedir")
+	if location == "" {
+		location = cfg.HttpProxyRedirURL
+	}
 
 	if location == "" {
 		host, _, err := net.SplitHostPort(c.Request.Host)