Initial support for blog posts completed.

* Posts now optionally accept constructor parameters to set attributes,
  e.g. `:title`;
* Method `Blog#new_post` now accepts such parameters to be passed to the
  Post constructor;
* The Blog :index view now displays a list of posts (if any).

RSpec: 53 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 16 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 0
Adding PlaceholderBuilder domain-service object, or DSO.

RSpec: 54 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 18 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 0
Use `PlaceholderBuilder` DSO in `BlogController#index` method.

Also feature spec to verify placeholder entries.

RSpec: 56 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 19 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 0