Add options for setup/teardown for whole file and per step

We probably should add options like:

- SUITE-SETUP/TEARDOWN: to be executed once when the markdown is first read and after the last step is executed
- STEP-SETUP/TEARDOWN: to be executed just before and after each step

This follows common patterns in unit testing frameworks. Today one has to hack around this.

The issue names no files or tests. Start by locating the markdown-reading and step-execution entry points, then trace where lifecycle options would be parsed and invoked. Done means suite and per-step setup/teardown options have defined execution timing and are covered by tests.

Written by the indexing model from the issue text.