chore: refactor codebase to promises

lib/command.js

@@ -2,10 +2,30 @@
 
 const betterStackTraces = /development/i.test(process.env.NODE_ENV) || /\bredis\b/i.test(process.env.NODE_DEBUG)
 
-function Command (command, args, callback, callOnWrite) {
+// TODO: Change the arguments to an object
+// callOnWrite could be two things now
+function Command (command, args, callOnWrite, transformer) {
   this.command = command
   this.args = args
   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
   if (betterStackTraces) {