Change posts style

* Replace posts italic's styling * with _
* Replace posts file extension makedown with md
Change HTML lang from zh-cmn-Hans to cmn-hans

The language tag zh-cmn-Hans is deprecated. Use cmn-hans instead.
Remove markdown: kramdown from _config

Kramdown is Jekyll's default Markdown converter.
Rename `class` wrap to wrapper
Change syntax.css to syntax.scss

* Nest CSS selectors
* Replace /* */ with //
Specify the output style (:compressed) for Sass
Replace blue color

* Replace #0c84e8 with #0091ea
Use Sass `@import`

* Rename style.sass to main.sass
Tweak post content style
Add font weight normal to `h1-h6`
Tweak post content

* Remove posts italic's styling
* Replace () with （）
Add hyperlink to technical terminology.