Added new PostsController and its "New Post" form.

This required adding new route to `config/routes.rb`.

Also added four custom matchers used by `new.slim_spec.rb`.

RSpec: 77 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 27 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 5