Enhancements

Fix not adding local rulesets as Clash rule-providers.
Add "nullnode" as node placeholder to export configurations without adding any real node.
Add support for using !!import in source subscription URLs.
Add support for allowInsecure and tfo argument in Trojan links.
Optimize codes.

src/multithread.cpp

@@ -74,7 +74,7 @@ void safe_set_times(string_array &data)
 std::shared_future<std::string> fetchFileAsync(const std::string &path, const std::string &proxy, int cache_ttl, bool async)
 {
     std::shared_future<std::string> retVal;
-    if(fileExist(path))
+    if(fileExist(path, true))
         retVal = std::async(std::launch::async, [path](){return fileGet(path, true);});
     else if(isLink(path))
         retVal = std::async(std::launch::async, [path, proxy, cache_ttl](){return webGet(path, proxy, cache_ttl);});