add extensible DebugOptions class

[t-vi]

This intends to make debug options more discoverable:

• dynamic registration,
• automatic docstring support,
• also included in documentation.

[lantiga]

Looks great.

At first I thought we could provide a singleton that one could modify for better discoverability, but ultimately having objects with combinations of options I can store and swap during debugging is going to be a lot more effective.