Generated a `Blog::PostDecorator` and updated the model definition.

Note that until/unless `Blog::Post` is an ActiveRecord or Mongoid model, we need to explicitly decorate the Post *after instantiation*; i.e., it can't be done from the constructor, and there are good reasons why it shouldn't be.

RSpec: 113 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 42 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 4
RuboCop local overrides in source and own specs: 1