Be consistent with indentation
Fix dividing by zero

(Never thought I'd ever write this aha)
Simplify `file_path` method
Move `root_directory` method to Generator::Base
Convert configuration.root to a pathname in the `root_directory` method
Create relative URLs instead of absolute ones

Thanks to @jlbribeiro for his persistence in convincing me this had to be
done (and it had to be!) <3
Use Ruby's ||= operator to cache the @file_directory instance variable