Move defaults found in config.rb to DEFAULTS Hash

The `Site` class was using short-circuit evaluation to set defaults. I consolidated these defaults in the `DEFAULTS` hash. Same type of problem as the one found in #935. I didn't write any new tests b/c this "theoretically" shouldn't change anything. :see_no_evil: 

Notes:
* Can we group the DEFAULTS hash into meaningful subdivisions (directory settings, server/url settings, rendering settings, etc.)?
* `show_drafts` and `limit_posts` must be set to `nil` by default or things break. Why not `false`? I'll investigate.
* Can we possibly extract a `Config` class to collaborate with `Site` so that `Site` can focus on creating the `_site` directory? (Probably not for 1.0.0)
* What's the difference between `include` and `keep_files`?