import github-markdown.css

from https://github.com/sindresorhus/github-markdown-css .
it is released under the MIT License.
set previm to use the css.
delete most part of origin.css

Because github-markdown.css has replaced it, most part of the
origin.css is no longer needed.
use Ricty in <pre> or <code> tags

for personal reasons, I'd like to use Ricty in <pre> and <code>.