Use in your _config.yml:

markdown: kramdown

kramdown:
  input: GFM
  hard_wrap: false

Test Markdown GFM - Code Block / Preformatted Block using Backticks

  ALICE'S RIGHT FOOT, ESQ.
    HEARTHRUG,
      NEAR THE FENDER,
        (WITH ALICE'S LOVE).

# Ruby knows what you
# mean, even if you
# want to do math on
# an entire Array
cities  = %w[ London
              Oslo
              Paris
              Amsterdam
              Berlin ]
visited = %w[Berlin Oslo]

puts "I still need " +
     "to visit the " +
     "following cities:",
     cities - visited

Test Markdown GFM - Hard Line Breaks Turned Off (Again)

Alice was beginning to get very tired of sitting by her sister on the bank, and of having nothing to do: once or twice she had peeped into the book her sister was reading, but it had no pictures or conversations in it, ‘and what is the use of a book,’ thought Alice ‘without pictures or conversations?’

So she was considering in her own mind (as well as she could, for the hot day made her feel very sleepy and stupid), whether the pleasure of making a daisy-chain would be worth the trouble of getting up and picking the daisies, when suddenly a White Rabbit with pink eyes ran close by her.