Feature: Obtain gist css url from API call

[weirdpattern]

The current API call returns a link to the stylesheet associated to the gist (see property stylesheet below).

{
  "description":".2016.08.31.stream-redirection",
  "public":true,
  "created_at":"2018-06-28T21:56:26.000-05:00",
  "files":[".2016.08.31.stream-redirection","example.sh","syntax.text"],
  "owner":"weirdpattern",
  "div":"...",
  "stylesheet":"https://assets-cdn.github.com/assets/gist-embed-87673c31a5b37b5e6556b63e1081ebbc.css"
}

The plugin should use this instead of a hardcoded link or a configuration setting.

[weirdpattern]

A work is already in progress in #9