feat: --include flag to prepend include.css

[andesco]

added --include boolean flag to CLI options:

• reads include.css from package directory;
• replaces root_selector placeholders in include.css with actual root selector value; and
• prepends generated stylesheets with include.css

example: --remote-selector=body

body {
  box-sizing: border-box;
  min-width: 200px;
  max-width: 980px;
  margin: 0 auto;
  padding: 45px;
}
@media (max-width: 767px) {
  body {
    padding: 15px;
  }
}

generated CSS examples: http://github.com/andesco/github-markdown-css-nova

[sindresorhus]

This is too specific to your use-case. It doesn't really belong here. You can do the same after generating the CSS file.