Improved interactive alias specifications.

Split interactive alias names and expansions into proper keys and
values, which has the demonstrable benefit of permitting subsequently
defined interactive alias names to overwrite previous key-value pairs.
Replaced all calls to say_first_section() and say_next_section() with
print_heading(). Documented a future roadmap for extending heading-style
groups to other output formats (e.g., messages, warnings, errors).

"The starry heaven's locke that trod
 the wedlock of
 the formless demiurge
 gurgles wicked tresses,
 Incessantly."