Add _config.yml parameters for sorting and excluding projects

[tterb]

I created a _config.yml parameter that allows the user to specify whether they would like to sort project cards by stars or pushed date, in response to @brandonrosage comment on #12 and the variety of proposed sorting preferences.

Additionally, I also added _config.yml parameters that allow the user to exclude forks as proposed in #19 and specify certain projects to exclude, in the case of inactive or no longer maintained projects.

[brandonrosage]

I just tightened up the YAML structure and taxonomy so that everything was under the banner of "projects":

projects:
  sort_by: pushed
  # sort_by options:
  #   - pushed
  #   - stars
  limit: 9
  exclude:
    forks: true
    projects:
    # - repo-name

I also added the limit property to this model.

Otherwise, this is a killer improvement!