Support username:pass authority in HTTP/WS urls

PUBLISHED_FROM=39a1c2a271c5cd961670e11c830105c17ba0b2e4

docs/c-api/http.h/mg_send_websocket_handshake3.md

@@ -0,0 +1,13 @@
+---
+title: "mg_send_websocket_handshake3()"
+decl_name: "mg_send_websocket_handshake3"
+symbol_kind: "func"
+signature: |
+  void mg_send_websocket_handshake3(struct mg_connection *nc, const char *path,
+                                    const char *host, const char *protocol,
+                                    const char *extra_headers, const char *user,
+                                    const char *pass);
+---
+
+Like mg_send_websocket_handshake2 but also passes basic auth header 
+