Add Gt, Gte, Lt, Lte operators

[asdine]

Same behaviour as

regula/rule/eq_expr.go

Lines 8 to 14 in b79b8ed

	func init() {
	Operators["eq"] = func() Operator { return newExprEq() }
	Operators["lt"] = func() Operator { return newExprLT() }
	Operators["gt"] = func() Operator { return newExprGT() }
	Operators["lte"] = func() Operator { return newExprLTE() }
	Operators["gte"] = func() Operator { return newExprGTE() }
	}