More #summarise_blog cleanup.

We really are getting somewhere here; this needs to move.

RSpec: 841 examples, 0 failures (but see Issue #87)
RuboCop: 161 files inspected, no offenses detected
Decoupled BlogSummariser from its base logic.

New `BaseSummariser` class has all the core logic and DSL support of
the previous `BlogSummariser`, with no application-specific (Blog- or
PostData-related) knowledge.

RSpec: 841 examples, 0 failures (but see Issue #87)
RuboCop: 161 files inspected, no offenses detected