add valine comment system

[xCss]

• Add valine comment system.
• Add comment system judgment.

_config.yml

# Valine comment https://valine.js.org
valine:
  appid:  # your leancloud appid
  appkey:  # your leancloud appkey
  notify: false # true/false:mail notify !!!Test,Caution. https://github.com/xCss/Valine/wiki/Valine-%E8%AF%84%E8%AE%BA%E7%B3%BB%E7%BB%9F%E4%B8%AD%E7%9A%84%E9%82%AE%E4%BB%B6%E6%8F%90%E9%86%92%E8%AE%BE%E7%BD%AE
  verify: false # true/false:verify code
  avatar: mm # avatar style https://github.com/xCss/Valine/wiki/avatar-setting-for-valine
  placeholder: Just go go # comment box placeholder

layout/_partial/comments.swig

    {% elseif theme.valine.appid and theme.valine.appkey %}
      <div id="vcomments"></div>
    {% endif %}

layout/_script/_comments/valine.swig

{% if theme.valine.appid and theme.valine.appkey %}
<!-- valine Comments -->
<script src="//cdn1.lncld.net/static/js/3.0.4/av-min.js"></script>
<script src="//cdn.jsdelivr.net/gh/xcss/valine@v1.1.7/dist/Valine.min.js"></script>
<script type="text/javascript">
    new Valine({
        el: '#vcomments',
        notify: {{ theme.valine.notify }},
        verify: {{ theme.valine.verify }},
        app_id: "{{ theme.valine.appid }}",
        app_key: "{{ theme.valine.appkey }}",
        placeholder: "{{ theme.valine.placeholder }}",
        avatar: '{{ theme.valine.avatar }}'
    });
</script>
{% endif %}

layout/_script/comments.swig

Add comment system judgment.

  {% if theme.disqus_shortname %}
    {% include "_comments/disqus.swig" %}
  {% elseif theme.changyan and theme.changyan.appid and theme.changyan.appkey %}
    {% include "_comments/changyan.swig" %}
  {% elseif theme.valine.appid and theme.valine.appkey %}
    {% include "_comments/valine.swig" %}
  {% endif %}