Added mg_websocket_write(), per morgan3d pull request

mongoose.h

@@ -208,6 +208,28 @@ struct mg_request_info *mg_get_request_info(struct mg_connection *);
 int mg_write(struct mg_connection *, const void *buf, size_t len);
 
 
+// Send data to a websocket client wrapped in a websocket frame.
+// It is unsafe to read/write to this connection from another thread.
+// This function is available when mongoose is compiled with -DUSE_WEBSOCKET
+//
+// Return:
+//  0   when the connection has been closed
+//  -1  on error
+//  >0  number of bytes written on success
+int mg_websocket_write(struct mg_connection* conn, int opcode,
+                       const char *data, size_t data_len);
+
+// Opcodes, from http://tools.ietf.org/html/rfc6455
+enum {
+  WEBSOCKET_OPCODE_CONTINUATION = 0x0,
+  WEBSOCKET_OPCODE_TEXT = 0x1,
+  WEBSOCKET_OPCODE_BINARY = 0x2,
+  WEBSOCKET_OPCODE_CONNECTION_CLOSE = 0x8,
+  WEBSOCKET_OPCODE_PING = 0x9,
+  WEBSOCKET_OPCODE_PONG = 0xa
+};
+
+
 // Macros for enabling compiler-specific checks for printf-like arguments.
 #undef PRINTF_FORMAT_STRING
 #if _MSC_VER >= 1400