Add bash-completion definition file
Add bash-completion file to install target
Add new configure option, --with-bashcompletiondir
Merge branch 'feature-bash-completion' into feature-zsh-completion
Add zsh completion file for img2sixel
Add a zsh completion file to install target
Add new configure option, --with-zshcompletiondir
Merge branch 'feature-zsh-completion' into completion