Detecting renamed files

## File rename detection
Coursera professors often re-organize course content while a course is running and this leads to changes in topic / unit names, which leads to coursera-dl downloading the same content again under different names.

This change is aimed to detect renames and reuse already downloaded files. Directory renames are not detected at this moment.

## A separate deduplication project
I've also created a separate project for removing duplicates in already downloaded content. This one is capable of detecting directory renames. Feel free to include this in your project or just put a link in your project description.
https://github.com/ilfats/dedup.git