move promiseModule argument into an option

fixes #28

index.js

@@ -31,13 +31,10 @@ var processFn = function (fn, P, opts) {
 	};
 };
 
-var pify = module.exports = function (obj, P, opts) {
-	if (typeof P !== 'function') {
-		opts = P;
-		P = Promise;
-	}
-
+var pify = module.exports = function (obj, opts) {
 	opts = opts || {};
+
+	var P = opts.promiseModule || Promise;
 	var exclude = opts.exclude || [/.+Sync$/];
 
 	var filter = function (key) {

readme.md

@@ -34,7 +34,7 @@ pify(fs).readFile('package.json', 'utf8').then(data => {
 
 ## API
 
-### pify(input, [promiseModule], [options])
+### pify(input, [options])
 
 Returns a promise wrapped version of the supplied function or module.
 
@@ -44,14 +44,6 @@ Type: `function`, `object`
 
 Callback-style function or module whose methods you want to promisify.
 
-#### promiseModule
-
-Type: `function`
-
-Custom promise module to use instead of the native one.
-
-Check out [`pinkie-promise`](https://github.com/floatdrop/pinkie-promise) if you need a tiny promise polyfill.
-
 #### options
 
 ##### multiArgs
@@ -113,6 +105,14 @@ if (promiseFn()) {
 }
 ```
 
+##### promiseModule
+
+Type: `function`
+
+Custom promise module to use instead of the native one.
+
+Check out [`pinkie-promise`](https://github.com/floatdrop/pinkie-promise) if you need a tiny promise polyfill.
+
 
 ## License
 

test.js

@@ -37,7 +37,7 @@ test('pass argument', async t => {
 });
 
 test('custom Promise module', async t => {
-	t.is(await fn(fixture, pinkiePromise)(), 'unicorn');
+	t.is(await fn(fixture, {promiseModule: pinkiePromise})(), 'unicorn');
 });
 
 test('multiArgs option', async t => {