Description

This PR adds .livemd extension to Markdown language.

.livemd is the extension of Livebook which is an Elixir version of Jupyter.
.livemd is a subset of Markdown.

.livemd can be displayed as Markdown on GitHub when .livemd has <!-- vim: syntax=markdown --> at the first line of it (Example), but it's not a default option of .livemd.

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?q=extension%3Alivemd&type=Code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s): x (I made it including all features of Livebook)
  • I have included a change to the heuristics to distinguish my language from others using the same extension.