Finished (first whack at) `DescribeBlog` interactor.

Returns (currently hardcoded) title and subtitle for "blog" per the book.

RSpec: 32 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 13 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2