More style fixes

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).

I will probably squash this PR into a single commit, since it gets quite noisy otherwise.
@mpapis The style guide is derived from practices found in many Ruby projects. It’s middle ground; it will never please everyone perfectly.

I started preferring consistency over doing the “right thing”. Consistency means that people know what to expect. The “right thing” is often not clear, and what the “right thing” is also differs from person to person.

From my experience at work dealing with a lot of Ruby projects written by many people, I find that Rubocop defines pretty nicely what my colleagues have in mind of what good Ruby code should look like. I’ve cast my personal preferences aside and went with a style that everyone agreed upon.

A code base with a consistent style is a pleasure to work with. While cleaning up the nanoc source code (mostly automatically, using Rubocop’s `--auto-correct`), I found many style quirks that clearly indicate that different people with different stylistic preferences have worked on the nanoc code base. Having Rubocop to automatically check the style (and perhaps correct it) makes sure the code base’s style remains consistent within the project itself.

That said, I don’t agree with everything that Rubocop says; for instance, I do not agree with `AsciiComments` (“Use only ASCII symbols in comments”), and that is why it is turned off for nanoc.