保存字符串到文件string.save

string.save("/log/log.txt",'');
string.save("/log/log.txt",'aaaaaaaa\n',true)
string.save("/log/log.txt",'bbbb\n',true)
string.save("/log/log.txt",'cccc\n',true)
//可用于写入日志文件
//自动创建父级目录
//带true时，为追加字符