excerpt.rb

module Jekyll

  class Excerpt < Liquid::Block
    def render(context)
      # Get the current post's post object
      id = context["page"]["id"]
      posts = context.registers[:site].posts
      post = posts [posts.index {|post| post.id == id}]

      # Put the block contents into the post's excerpt field,
      # and also return those contents
      post.data["excerpt"] = super
    end
  end

end

Liquid::Template.register_tag('excerpt', Jekyll::Excerpt)

how-tousage.rb

{% for post in site.posts %}
    {% if post.excerpt %}
        {{ post.excerpt | markdownify }}
    {% else %}
        {{ post.content }}
    {% endif %}
{% endfor %}