Add an explain option to the following commands:

• remove() (MDB 3.0+)
• update() (MDB 3.0+)
• distinct() (MDB 3.2+)
• findAndModify() (MDB 3.2+)
• mapReduce() (MDB 4.4+)

The explain option should support a boolean or string.

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