feat: add the configure for declare copyright on posts

[Jacksgong]

Config on the _config.yml

# Declare license on posts
post_copyright:
  enable: true
  license: CC BY-NC-SA 3.0 CN
  license_url: http://creativecommons.org/licenses/by-nc-sa/3.0/cn/

Result

[Jacksgong]

You can check out the effect from my blog.

[iissnan]

Thanks. 👍

[ivan-nginx]

There is some questions about this pull.

In _config.yml:

1. Mb post_copyright in config need to replace into post block?
2. Why CN is default? EN for defualt must be always.
3. license_url must be httpS.

In post-copyright.swig:

1. Why style is not use note tag style? There is no customization.
2. Why CN langauge again?
3. And where is customization of language again?

@Jacksgong i undestand what this is your own customization, but need to do for all peoples and all languages, not only for CN.

@iissnan very easy merge.

[iissnan]

@ivan-nginx Good catches. I am working on the improvement to this pr right now.

[xiao-fang]

how can i disable some posts' copyrights but not all posts? via font-matter?