Maintain at most 1 newline between Liquid branches

[charlespwd]

Sometimes, to enhance readability, it is preferabble to have newlines between branches.

Prettier does that for JS switch and case statements, we now do the same.

{% case foo %}
  {% when bar %}
    <div>Something</div>

  {% when baz %}
    <div>Something else</div>
{% endcase %}

Fixes #132