Add .coffee.md to Literate Coffeescript

[sheerun]

Description

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:

It seems github search is broken for multiple-part extensions like .coffee.md as it doesn't return any results (even the provided sample): https://github.com/search?q=extension%3Acoffee.md+require&type=Code

• I have included a real-world usage sample for all extensions added in this PR:
  • Sample source(s):
    https://github.com/STRd6/pixi-test/blob/master/main.coffee.md
  • Sample license(s):
    https://github.com/STRd6/pixi-test/blob/master/LICENSE
• I have included a change to the heuristics to distinguish my language from others using the same extension.
  Does not apply

[lildude]

It seems github search is broken for multiple-part extensions like .coffee.md as it doesn't return any results (even the provided sample): https://github.com/search?q=extension%3Acoffee.md+require&type=Code

It's not perfect, but we can get closer with path:coffee extension:md require which in this case only brings up 28 results.

Switching out require for NOT nothack returns only 900-odd results, many of which are legit markdown files so I don't think we can include this just yet.

[sheerun]

Google Bigquery for:

SELECT COUNT(*) FROM (
  SELECT repo_name FROM `bigquery-public-data.github_repos.files`
  WHERE ENDS_WITH(path, ".coffee.md")
  GROUP BY repo_name
)

returns 1250 repositories (not files), which have such extensions, for github in 2015. So I guess it qualifies?

[lildude]

Ah yes. Of course. I forgot BigQuery could search for cases like that. Yup. Looks like we've got the numbers.