Implement Allowed Directory List

[mfocuz]

This library can be built into application(e.g. admin portal of some webapp, that is the case I encountered).

While reading log files is a feature of app for troubleshooting purposes, it should not allow to read files from arbitrary directories.
Request log file path should be normalized and checked if it is still inside of allow directory(or list of directories).

Otherwise it can lead to security issue CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')