Add Waline comment platform with theme switching

[felixnie]

This PR added Waline to the list of commenting systems available for Hydejack.

Before you start, you may consider changing the layout of the _config.yml file for simple switching between commenting systems:

# Setting a disqus shortname will enable the comment section on
# pages with `comments: true` in the front matter.
# disqus:                felixnie

# EDIT: include more comment providers.
comments:
  # 'disqus'/'commento'/'giscus'/'waline'
  provider:            waline

  disqus:
    # If you wanna continue using disqus, you may want to change the way it finds its shortname in _config,yml.
    shortname:         felixnie

  giscus:
    ...
  
  commento:
    ...

  waline:
    server:            "https://comment.felixnie.com"
    locale:            "zh-CN"

To make Waline render the right color after page refresh, add the missing label to body: Added Waline theme switching for Hydejack

The overwritten CSS makes it match the default theme of Hydejack. Change it to whatever you like.

Compared to Disqus:

1. fast loading time
2. support self-hosting
3. can comment without login
4. rich extensions
5. ...