Improve heading command matching logic.
Assert on parsed subcommand.
Work on supporting subcommands.
Work on subcommand support.

Dropping instantiation, define module should convert.
Remove preamble.
Update define dependency.

Loaded programs now define nested options and commands that are of the
correct class.
Tidy unused format.
Fix section concatenation issue.
Return on comment tokens in renderer.
Improve trailing whitespace on sections text.
0.1.17
Update test fixture.
Draft deprecation warnings.
Move deprecated modules to legacy object.
0.1.18
Fix tests.
0.1.19