Better support for loading from config file

[0xdevalias]

The standard plugins (such as jekyll-feed) are loadable in config such as jekyll-feed, but due to the directory layout, we need to do it as jekyll/tagging. It would be cool to make this match.

_config.yml

gems:
  - jekyll/tagging          # https://github.com/pattex/jekyll-tagging
  - jekyll-feed             # https://github.com/jekyll/jekyll-feed

Additionally, updating the install instructions to show the config method would be awesome.