Add post navigation display configuration

[liolok]

PR Checklist

• The commit message follows our guidelines.
• Tests for the changes was maked (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs in NexT website have been added / updated (for features).

PR Type

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Build related changes.
• CI related changes.
• Documentation content changes.
• Other... Please describe:

What is the current behavior?

Theme displays post navigation (previous post and next post) in post footer by default.

What is the new behavior?

Theme displays post navigation (previous post and next post) in post footer by default, and provides an option to turn it off.

How to use?

In NexT _config.yml:

...
post_navigation: true # set it to false to disable navigation display

Does this PR introduce a breaking change?

• Yes.
• No.

[stevenjoezhang]

Please, no more useless options. Please.

[jiangtj]

Please, no more useless options. Please.

我不认为这是个无用的选项，因为上下页的功能是内置，提供选项禁用也是合理的（虽然可能很少有人用到）.....

I don't think it's a useless option, because the functions of the previous and next pages are built-in, and it's reasonable to provide option disable (although it may be rarely used)

[stevenjoezhang]

因为上下页的功能是内置，提供选项禁用也是合理的

「内置」和「提供选项禁用」没有任何逻辑关联。

作为一个开源主题，用户拥有修改源码的自由，这就足够按自己的喜好实现功能了。诸位应该清楚的认识到，NexT主题1000多行配置就是由于过去几年中过于纵容用户，将许多与NexT主题无关的内容强行加进来，甚至不止一次的给Hexo背锅。如果可以的话，让Hexo出一个post_navigation helper，最好还能支持调整左、右，我举双手赞成。
以上。

[stevenjoezhang]

Superseded by 1417248

iblh/hexo-theme-material#288
hexojs/hexo#2262 (comment)