Add support for the list flag in bump-packages script

[victorpopkov]

Problem

Similar to dstmodders/docker-imagemagick#40:

It would be nice to have a -l, --list flag that will only list packages with their current versions as found inside Dockerfiles. This would make the script testing process easier when dealing with situations like dstmodders/docker-imagemagick#37.

Proposed solution

Add support for the -l, --list flag in the bin/bump-packages.sh script that will only list packages and their current versions.

Progress

• Add support for the FLAG_LIST variable and setting its value through the argument parser.
• Change bin/bump-packages.sh header to include the new flag in help.
• Change update_alpine_dockerfile() and update_debian_dockerfile() to implement the new behaviour.