css command

Command css appends CSS styles to content/static/stylesheet.css. It reads from the CSS file at https://github.com/sindresorhus/github-markdown-css/blob/gh-pages/github-markdown.css and removes all styles that do not belong to a .markdown-body <tag>. The .markdown-body class is then replaced with .Overview-readmeContent, for use in the discovery codebase. The remaining properties are written to content/static/css/stylesheet.css.