Adapt 'bak' for files and directories

The 'bak' function checks if it is a directory or a file and create a backup for
each case.
Improve configuration of diff for Git
Update 'oh-my-zsh' plugin for the rebase
Make 'zsh-update' file into the repository

The '.zsh-update' file is used by 'oh-my-zsh' to know the last update time.
Add a script to download videos of a youtuber

This script will download all videos of a youtuber. Just find the Youtube
webpage and download the HTML file (look at '--help' for more details).
Make the 'update.sh' executable

The 'update.sh' script (to update all dot-files links) has been made executable.
Fix the test in shell scripts

The 'test' command in shells is now written as '[ ]'.
Fix the parsing of Youtube HTML webpage

The script should only select Youtube web links where '/watch' is into it and
filter others hyperlinks.
Rename 'youtuber.sh' script into 'youtuber'
Remove an non useful commented line
Add the ZSH script directory to the PATH
Add a Image Magick script to remove dirt

This script can remove a dirt on a photography.