Completed the "Adding timestamps" chapter (Section 9).

This involved adding a `pubdate` property to the `Post` class, and changing the signature of `Post#publish` to inject a publication date. Also, obviously, adding the `pubdate` property to the output from the `_entry` partial.

RSpec: 76 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 28 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 4
RuboCop local overrides in source and own specs: 1