feat: add some article-related functions

These functions include get_article_excerpt and get_article_words.

_includes/functions/get_article_excerpt.html

@@ -0,0 +1,35 @@
+{% if include.params.article %}
+  {% assign article = include.params.article %}
+{% endif %}
+
+{% if include.params.lang %}
+  {% assign lang = include.params.lang %}
+{% else %}
+  {% assign lang = lang | default: site.lang | default: "en" %}
+{% endif %}
+
+{% if include.params.excerpt_size %}
+  {% assign excerpt_size = include.params.excerpt_size %}
+{% else %}
+  {% assign excerpt_size = excerpt_size | default: 800 %}
+{% endif %}
+
+{%- include functions.html func='get_article_words' -%}
+{% assign words = return %}
+
+{% assign _article = article | strip_html %}
+{% assign _words = _article | size %}
+
+{% if lang != "en" %}
+  {% assign _size = words
+    | times: 1.0
+    | divided_by: _words
+    | times: excerpt_size
+    | round %}
+{% endif %}
+
+{% if _size > excerpt_size %}
+  {% assign _size = excerpt_size %}
+{% endif %}
+
+{% assign return = _article | truncate: _size %}

_includes/functions/get_article_words.html

@@ -0,0 +1,26 @@
+{% if include.params.article %}
+  {% assign article = include.params.article %}
+{% endif %}
+
+{% if include.params.lang %}
+  {% assign lang = include.params.lang %}
+{% else %}
+  {% assign lang = lang | default: site.lang | default: "en" %}
+{% endif %}
+
+{% assign words = article | number_of_words %}
+
+{% if lang != "en" %}
+  {% assign words = words
+    | times: 0.6
+    | round %}
+  {% assign words = article
+    | strip_html
+    | strip_newlines
+    | size
+    | times: 0.4
+    | plus: words
+    | round %}
+{% endif %}
+
+{% assign return = words %}