Support named groups in "Supported tags"

I was thinking today that it would be interesting to support named groups in the "Supported tags" lists in the README.md files.

For example, given a library file like this:

# maintainer: InfoSiftr <github@infosiftr.com> (@infosiftr)

# group: PHP 5.6
5.6.6-cli: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.6
5.6-cli: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.6
5-cli: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.6
cli: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.6
5.6.6: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.6
5.6: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.6
5: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.6
latest: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.6

5.6.6-apache: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.6/apache
5.6-apache: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.6/apache
5-apache: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.6/apache
apache: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.6/apache

5.6.6-fpm: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.6/fpm
5.6-fpm: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.6/fpm
5-fpm: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.6/fpm
fpm: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.6/fpm

# group: PHP 5.5
5.5.22-cli: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.5
5.5-cli: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.5
5.5.22: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.5
5.5: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.5

5.5.22-apache: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.5/apache
5.5-apache: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.5/apache

5.5.22-fpm: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.5/fpm
5.5-fpm: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.5/fpm

# group: PHP 5.4
5.4.38-cli: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.4
5.4-cli: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.4
5.4.38: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.4
5.4: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.4

5.4.38-apache: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.4/apache
5.4-apache: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.4/apache

5.4.38-fpm: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.4/fpm
5.4-fpm: git://github.com/docker-library/php@1e07e9134490f53d24fbd23ee3521067c8ab2d29 5.4/fpm

You'd get the following YAML markup:

- **PHP 5.6**
  - [`5.6.6-cli`, `5.6-cli`, `5-cli`, `cli`, `5.6.6`, `5.6`, `5`, `latest` (*5.6/Dockerfile*)](https://github.com/docker-library/php/blob/1e07e9134490f53d24fbd23ee3521067c8ab2d29/5.6/Dockerfile)
  - [`5.6.6-apache`, `5.6-apache`, `5-apache`, `apache` (*5.6/apache/Dockerfile*)](https://github.com/docker-library/php/blob/1e07e9134490f53d24fbd23ee3521067c8ab2d29/5.6/apache/Dockerfile)
  - [`5.6.6-fpm`, `5.6-fpm`, `5-fpm`, `fpm` (*5.6/fpm/Dockerfile*)](https://github.com/docker-library/php/blob/1e07e9134490f53d24fbd23ee3521067c8ab2d29/5.6/fpm/Dockerfile)
- **PHP 5.5**
  - [`5.5.22-cli`, `5.5-cli`, `5.5.22`, `5.5` (*5.5/Dockerfile*)](https://github.com/docker-library/php/blob/1e07e9134490f53d24fbd23ee3521067c8ab2d29/5.5/Dockerfile)
  - [`5.5.22-apache`, `5.5-apache` (*5.5/apache/Dockerfile*)](https://github.com/docker-library/php/blob/1e07e9134490f53d24fbd23ee3521067c8ab2d29/5.5/apache/Dockerfile)
  - [`5.5.22-fpm`, `5.5-fpm` (*5.5/fpm/Dockerfile*)](https://github.com/docker-library/php/blob/1e07e9134490f53d24fbd23ee3521067c8ab2d29/5.5/fpm/Dockerfile)
- **PHP 5.4**
  - [`5.4.38-cli`, `5.4-cli`, `5.4.38`, `5.4` (*5.4/Dockerfile*)](https://github.com/docker-library/php/blob/1e07e9134490f53d24fbd23ee3521067c8ab2d29/5.4/Dockerfile)
  - [`5.4.38-apache`, `5.4-apache` (*5.4/apache/Dockerfile*)](https://github.com/docker-library/php/blob/1e07e9134490f53d24fbd23ee3521067c8ab2d29/5.4/apache/Dockerfile)
  - [`5.4.38-fpm`, `5.4-fpm` (*5.4/fpm/Dockerfile*)](https://github.com/docker-library/php/blob/1e07e9134490f53d24fbd23ee3521067c8ab2d29/5.4/fpm/Dockerfile)

And the following output:

• PHP 5.6
  • 5.6.6-cli, 5.6-cli, 5-cli, cli, 5.6.6, 5.6, 5, latest (5.6/Dockerfile)
  • 5.6.6-apache, 5.6-apache, 5-apache, apache (5.6/apache/Dockerfile)
  • 5.6.6-fpm, 5.6-fpm, 5-fpm, fpm (5.6/fpm/Dockerfile)
• PHP 5.5
  • 5.5.22-cli, 5.5-cli, 5.5.22, 5.5 (5.5/Dockerfile)
  • 5.5.22-apache, 5.5-apache (5.5/apache/Dockerfile)
  • 5.5.22-fpm, 5.5-fpm (5.5/fpm/Dockerfile)
• PHP 5.4
  • 5.4.38-cli, 5.4-cli, 5.4.38, 5.4 (5.4/Dockerfile)
  • 5.4.38-apache, 5.4-apache (5.4/apache/Dockerfile)
  • 5.4.38-fpm, 5.4-fpm (5.4/fpm/Dockerfile)

Start with the README.md "Supported tags" lists and the example library file in the issue. Compare the requested named-group YAML and rendered output with current behavior, and consider the work complete when groups such as PHP 5.6, 5.5, and 5.4 render with their associated tags and links.