feat: Add a configuration for an instance that can be set at creation time

And use it to control whether the result of a complex operation where the
   imaginary part comes out to 0 will be coerced back to a number.

test/custom.js

@@ -36,4 +36,14 @@ describe('Custom instances', () => {
       assert.strictEqual('subtract' in cherry, false)
       assert.strictEqual('negate' in cherry, false)
    })
+
+   const res = picoInstance('resolving', { resolveComplex: true })
+   createNumbers(res)
+   createComplex(res)
+
+   it("can be configured", () => {
+      assert.strictEqual(res.add(res.complex(2,3), res.complex(2,-3)), 4)
+      assert.deepStrictEqual(math.add(math.complex(2,3), math.complex(2,-3)),
+         math.complex(4,0))
+   })
 })