Add toc max_depth support

[jiangtj]

PR Checklist

Please check if your PR fulfills the following requirements:

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

PR Type

What kind of change does this PR introduce?

• 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?

Don't support set toc max_depth

Issue Number(s): #527

What is the new behavior?

Add toc max_depth support

• https://jiangtj.gitlab.io/next-test/muse/2018/12/22/max_depth_in_config/
• Config set max_depth: 2
• https://jiangtj.gitlab.io/next-test/muse/2018/12/22/max_depth_in_post/
• This post set toc_max_depth:1

How to use?

In NexT _config.yml:

toc:
  # Maximum heading depth of generated toc. You can set it in one post through `toc_max_depth` var.
  max_depth: 6

Or in post:

toc_max_depth: 6

Does this PR introduce a breaking change?

• Yes.
• No.

[ivan-nginx]

Add docs please to NexT website: https://github.com/theme-next/theme-next.org