Debug should be an interface button rather than a prelude option

[ClementSparrow]

...like verbose_logging.

[ClementSparrow]

Actually, it would also be nice to add some interface allowing users to see how a specific rule is expanded, instead of searching for it in the output of debug.

[ClementSparrow]

Reopening because the remark in my last comment is still valid.

[ClementSparrow]

Closing, now that #34 covers the remaining concerns.