Improve escaping string `\` and `"` in dhall configuration file about dhall-haskell

[Gabriella439]

I think the best solution here is to have Dhall provide support for Nix's "double single quote" string literals (except perhaps without antiquotation for the first draft)

[Gabriella439]

Alright, so with master you can now write:

{ stats = ''
    pepper --client=runner manage.status | jq '.return[0] | [.up , .up + .down | length] as $stats | {up, down, stats: "\($stats[0]) up of \($stats[1])"}'
  ''
}

... just like in Nix