implement a plugin system

[compscidr]

Need to think on this a bit mroe but we need some spots were people can tie in their own code without needing to fork the repo. I'm thinking we need:

• scheduled jobs
• things that run before we pass off to the template for
  • individual posts, pages
  • groups of posts
  • archives, tags
• ability to have plugin settings which show up in the admin interface somehow

existing things we likely want to turn into plugins:

1. social icons (move the settings out into per-plugin settings)
2. the scholar stuff
3. perhaps some type of google analytics plugin where the user just has to fill in their GA number.