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.

complex/add.js

@@ -2,6 +2,7 @@ import { anyComplex } from './complex.js'
 
 export default function create(pmath) {
     const complex = pmath('complex')
+    const resolve = pmath.config.resolveComplex
     return pmath('add', [anyComplex, // naive, but this is just a P-o-C
        (...addends) => {
            let sum = complex(addends[0])
@@ -10,6 +11,7 @@ export default function create(pmath) {
                sum.re += addend.re
                sum.im += addend.im
            }
+           if (resolve && Math.abs(sum.im/sum.re) < 1e-10) return sum.re
            return sum
        }])
 }

picomathInstance.js

@@ -1,7 +1,7 @@
 /* Core of picomath: generates an instance */
 import poortf from './poortf.js'
 
-export default function picomathInstance (instName) {
+export default function picomathInstance (instName, config) {
     /* Since we have to do it all the time, when we call a picomath instance
      * as a function, it takes a name and 0 or more implementations add adds
      * them to its poortf property named name, returning that property value.
@@ -16,6 +16,7 @@ export default function picomathInstance (instName) {
     }
 
     Object.defineProperty(fn, 'name', {value: instName})
+    fn.config = config || { resolveComplex: false } // primitive default for POC
 
     return fn
 }

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))
+   })
 })