Give lua access to flushing a directory cache

did not consider how this could go wrong but i needed this for one really specific time critical thing (the cache resets otherwise after 30 seconds)

src/RageUtil/File/RageFileManager.cpp

@@ -1295,13 +1295,20 @@ class LunaRageFileManager : public Luna<RageFileManager>
 		LuaHelpers::CreateTableFromArray(vDirs, L);
 		return 1;
 	}
+	static int FlushDirCache(T* p, lua_State* L)
+	{
+		auto dir = SArg(1);
+		p->FlushDirCache(dir);
+		return 0;
+	}
 
 	LunaRageFileManager()
 	{
 		ADD_METHOD(DoesFileExist);
 		ADD_METHOD(GetFileSizeBytes);
 		ADD_METHOD(GetHashForFile);
 		ADD_METHOD(GetDirListing);
+		ADD_METHOD(FlushDirCache);
 	}
 };