Refactored `Blog::AddingPosts#new_post` method.

Broke clunky parameter-parsing and text-post setup out into new classes `Blog::Support::PostParamParser` and `Blog::Support::TextPostBuilder` respectively.

RSpec: 112 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 40 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 4
RuboCop local overrides in source and own specs: 1
Source lines not covered by specs: 1