Add an explain option to the following cursor-based commands:

• aggregate() (MDB 3.0+)
• find() (MDB 3.0+)

The explain option should support a boolean or string.

• If a string, should specify the verbosity level; one of queryPlanner | queryPlannerExtended | executionStats
• If a boolean and true, should default to queryPlanner

In addition to supporting the option, these cursor-based commands should also provide a .explain builder method, which should accept an optional verbosity argument.