Fix many Rubocop offenses

This is the second part of my crusade against non-standard code style;
the first part being #476. There are two reasons why these changes were
not in #476:

1. Rubocop got updated with a lot of new checks, and this PR ensures we
   adhere to the guidelines set forth in that new Rubocop.

2. I got over the attachment to my personal preferences (such as TODO’s
   not being postfixed by a colon).
Merge pull request #500 from nanoc/more-style-fixes

More style fixes