fix: accept single function for middleware

* Fixed type check in .middleware() method

`if (typeof callback === 'object')` should be `if (typeof callback === 'function')`

See Issue #1214 for details

* middewareFactory accepts a function, not an object

According to [the docs for the `.middleware()` method](http://yargs.js.org/docs/#api-middlewarecallbacks):

> The `callbacks` parameter can be a function or a list of functions

See Issue #1214 for more details

* Added/Updated middleware tests

I added a new test to verify that the `.middleware()` method works when a single function is passed, rather than an array of functions.

Also enhanced an existing test to ensure that _all_ middleware is run before commands, regardless of whether it was added as an array of functions or a single function.

lib/middleware.js

@@ -2,7 +2,7 @@ module.exports = function (globalMiddleware, context) {
   return function (callback) {
     if (Array.isArray(callback)) {
       Array.prototype.push.apply(globalMiddleware, callback)
-    } else if (typeof callback === 'object') {
+    } else if (typeof callback === 'function') {
       globalMiddleware.push(callback)
     }
     return context

test/middleware.js

@@ -24,11 +24,30 @@ describe('middleware', () => {
   it('should add a single callback to global middleware', () => {
     const globalMiddleware = []
 
-    middlewareFactory(globalMiddleware)({})
+    middlewareFactory(globalMiddleware)(function () {})
 
     globalMiddleware.should.have.lengthOf(1)
   })
 
+  it('runs the middleware before reaching the handler', function (done) {
+    yargs(['mw'])
+      .middleware(function (argv) {
+        argv.mw = 'mw'
+      })
+      .command(
+        'mw',
+        'adds func to middleware',
+        function () {},
+        function (argv) {
+          // we should get the argv filled with data from the middleware
+          argv.mw.should.equal('mw')
+          return done()
+        }
+      )
+      .exitProcess(false) // defaults to true.
+      .parse()
+  })
+
   it('runs all middleware before reaching the handler', function (done) {
     yargs(['mw'])
       .middleware([
@@ -56,11 +75,9 @@ describe('middleware', () => {
 
   it('should be able to register middleware regardless of when middleware is called', function (done) {
     yargs(['mw'])
-      .middleware([
-        function (argv) {
-          argv.mw1 = 'mw1'
-        }
-      ])
+      .middleware(function (argv) {
+        argv.mw1 = 'mw1'
+      })
       .command(
         'mw',
         'adds func list to middleware',
@@ -69,12 +86,20 @@ describe('middleware', () => {
           // we should get the argv filled with data from the middleware
           argv.mw1.should.equal('mw1')
           argv.mw2.should.equal('mw2')
+          argv.mw3.should.equal('mw3')
+          argv.mw4.should.equal('mw4')
           return done()
         }
       )
+      .middleware(function (argv) {
+        argv.mw2 = 'mw2'
+      })
       .middleware([
         function (argv) {
-          argv.mw2 = 'mw2'
+          argv.mw3 = 'mw3'
+        },
+        function (argv) {
+          argv.mw4 = 'mw4'
         }
       ])
       .exitProcess(false) // defaults to true.