chore: add callback functionality back in

This also improves the performance for multi / batch commands a lot.
The reason is that now there are only callbacks internally even if
a promise is going to be returned in the end.

lib/command.js

@@ -2,36 +2,51 @@
 
 const betterStackTraces = /development/i.test(process.env.NODE_ENV) || /\bredis\b/i.test(process.env.NODE_DEBUG)
 
-// TODO: Change the arguments to an object
-// callOnWrite could be two things now
-function Command(name, args, callOnWrite, transformer) {
+function Command(name, args) {
+  var callback
+  if (args.length !== 0 && typeof args[args.length - 1] === 'function') {
+    this.promise = undefined
+    callback = args.pop()
+  } else {
+    this.promise = new Promise((resolve, reject) => {
+      callback = (err, res) => {
+        if (this.transformer !== undefined) {
+          const tmp = this.transformer(err, res)
+          err = tmp[0]
+          res = tmp[1]
+        }
+        if (err === null) {
+          resolve(res)
+        } else {
+          reject(err)
+        }
+      }
+    })
+  }
+  this.callback = callback
   this.command = name
   this.args = args
   this.argsLength = 0
   this.bufferArgs = false
-  var callback
-  transformer = transformer || function (err, res) {
-    return err || res
-  }
-  this.promise = new Promise((resolve, reject) => {
-    callback = (err, res) => {
-      if (err) {
-        const transformed = transformer(err)
-        if (transformed.stack) { // instanceof Error
-          reject(transformed)
-        } else {
-          resolve(transformed)
-        }
-      } else {
-        resolve(transformer(null, res))
-      }
-    }
-  })
-  this.callback = callback
-  this.callOnWrite = callOnWrite
+  this.transformer = undefined
+  this.callOnWrite = undefined
   if (betterStackTraces) {
     this.error = new Error()
   }
 }
 
-module.exports = Command
+function MultiCommand(name, args) {
+  this.command = name
+  this.args = args
+  this.argsLength = 0
+  this.bufferArgs = false
+  this.transformer = undefined
+  this.promise = undefined
+  this.callback = undefined
+  this.callOnWrite = undefined
+}
+
+module.exports = {
+  Command,
+  MultiCommand
+}