Add --config global option and blog_slug variable

1. Allow user to specify a configuration file other than `planet.yml` on the command line as a global option.
2. Create a variable `blog_slug` that slugifies the `blog_name` variable to be used in the `header.md` file.

(Slugify: turn "Tamouse's Extravagent Blog" into "tamouses-extravagent-blog")