Support YAML's .yaml.sed extension

[pchaigno]

This pull request adds support for YAML's .yaml.sed file extension. It seems to indicate YAML files in which a couple constants are expected to be "initialized" with sed.

Checklist:

• I am associating a language with a new file extension.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?o=desc&q=extension%3Ased+apiVersion&s=&type=Code (288 repositories by 238 users according to Harvester)
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s): coredns.yaml.sed
    • Sample license(s): Apache v2.0
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[Alhadis]

You've accidentally added an HTML file as the sample.

[pchaigno]

Uh, not again... Thanks @Alhadis! It's fixed.