Add Dhall language support.

[sellout]

Fixes dhall-lang/dhall-lang#198.

There’s not yet a Dhall grammar in a supported format, so it uses Haskell for now.

Description

Checklist:

• 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?utf8=%E2%9C%93&type=Code&ref=searchresults&q=extension%3Adhall+NOT+nothack
      • here are Harvester results:
        • Unique repos: 489
        • Unique users: 314
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/dhall-lang/dhall-lang/blob/master/tests/parser/success/largeExpressionA.dhall
      • https://github.com/dhall-lang/dhall-lang/blob/master/tests/normalization/success/remoteSystemsA.dhall
  • I have included a syntax highlighting grammar: https://github-lightshow.herokuapp.com/
  • I have updated the heuristics to distinguish my language from others using the same extension.

[f-f]

Looks like CI is failing due to the colour being too close to existing ones.
I cannot seem to get linguist set up on my machine so I'm not able to run the proximity test script, but here's some colour suggestions:

• #dd1a11
• #d1a11f
• #dfafff

[f-f]

@sellout update: I did get the tests running, and so far #dfafff is the only color that passed. Would that be fine?