helpers.cpp: Add new functions

- readFile()
  - parseJsonFromFile()
  - parseJsonFromString()

  parseJsonFromString() and parseJsonFromFile() use Json::CharReader
  instead of the obsolete function Json::Reader

src/system/helpers.h

@@ -152,4 +152,6 @@ std::string filehash(const char * file);
 std::string get_path(const char * path);
 inline bool file_exists(const std::string file) { return file_exists(file.c_str()); }
 
+std::string readFile(std::string file);
+
 #endif