lib/curl/Easy: add curl_easy_escape() wrapper

src/lib/curl/Easy.hxx

 /*
- * Copyright (C) 2016 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright (C) 2016-2018 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -87,6 +87,10 @@ public:
 		if (code != CURLE_OK)
 			throw std::runtime_error(curl_easy_strerror(code));
 	}
+
+	char *Escape(const char *string, int length=0) const noexcept {
+		return curl_easy_escape(handle, string, length);
+	}
 };
 
 #endif