Implement #10, add elementary tags/categories support.

- Currently, tags/categories only consider "posts".
- One post can have multiple tags, but can only belongs to one category.
- Using "#+TAGS: " and "+CATEGORY: " to specify tags/category of the
  post.
Add draft docs for feature design and implementation
docs format refinement, add two new feature drafts
Fix a TOC issue related to #3

If the org file doesn't contains a valid TOC, then set enable-toc to
nil, thus disable the toc div in the generated html page.