This PR will:

1. support toc: true property in article config (fix 为毛[TOC]不能用…… #65 and fix 请问 Ink 用的 blackfriday v1.5 可否升级为 v2 #94);
2. generate MathJax-friendly syntax. e.g.

   $ \LaTeX $
   $$ E = mc^2 $$

   now will be converted to

   <span class="math inline">\[ \LaTeX \]</span>
   <span class="math display">\[ E = mc^2 \]</span>

   (close 请增加katex支持 #95)
3. prevent pages with hide: true from being accessed by visible posts' Prev and Next (fix 隐藏问题 #108);
4. Prev & Next's URLs are wrongly escaped on Windows in post lists filtered by tags (fix Coding Pages 搭建 标签 不能翻页 /tag%5c写作%5cpage2.html #86).
   • p.s. any file paths passed into html/template should NOT have backslash delimiters, which is common on Windows.