Make extra stats have a flag and documentation.

	* bin/verilator: Document --extra-stats.
	* src/V3Options.cpp (void V3Options::parseOptsList): Add
	--extra-stats
	(V3Options::V3Options): likewise.
	* src/V3Options.h (class V3Options): Likewise.
	* src/V3Stats.cpp (StatsVisitor::visit(AstVar* nodep, AstNUser*)):
	Make extra stats dependent on --extra-stats.
	(StatsVisistor::~StatsVisitor): Likewise.