Change posts styles

* 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 // in syntax.scss