Added Mustache templates as a separate type (modelled on Latte)

[yourpropertyexpert]

Description

Added a single extra language type - Mustache, which previous versions of linguist (mis-)classified as HTML+Django.

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=mustache+extension%3Amustache&type=Repositories&ref=advsearch&l=&l=
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • Added to the /samples directory in a new directory called Mustache
    • Sample license(s): Apache 2, but given I wrote them for this submission, I'm happy for the github / linguist project to assign a different licence

• I am adding a new language.

  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension:
      • https://github.com/search?q=mustache+extension%3Amustache&type=Repositories&ref=advsearch&l=&l=
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • As above

• I am fixing a misclassified language

  • Only to the extent that the language's normal extension was incorrectly identified with another group

• I am adding new or changing current functionality

  • I have added or updated the tests for the new or changed functionality.
    • To the extent that I've added the new samples to the vendor README, which may have been the wrong thing to do

[stale]

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[yourpropertyexpert]

Alas, seems to have been overlooked.

[lildude]

LGTM