catalog-files.sh: Assign script argument to a variable
catalog-files.sh: Adjust regexp

We want to be able to catch 'description: ' in Markdown YAML headers
as well as '# Description' in shell script headers.
changelog.sh: Change git tag -list output format

- Change -n5 to -n99 for lines of tag annotation shown
- Strip all leading whitespace with sed
tag-release.sh: Add user message to temporary tag annotation file
update-readme.sh: Set readme and preview_app variables
update-readme.sh: Extend functionality

- Move some instructions into a function
- Create and preview file, with some dialogue, rather than just stdout