Find a config file by searching up directory tree from cwd to stopDir.
If config includes a path separator or is absolute, it is treated as a path. Otherwise it is treated as a filename and searched upwards.
Error if config file not found
Find a config file by searching up directory tree from cwd to stopDir.
If config includes a path separator or is absolute, it is treated as a path. Otherwise it is treated as a filename and searched upwards.