Refactor async parallel with Promise.all

Ref jrit#58

In this diff I refactored as small as possible part of code to replace
`async.parallel` with `Promise.all` and do not introduce any breaking
changes.

Further refactoring can be made in separate PR.

package.json

@@ -31,7 +31,6 @@
     "mocha": "^6.2.2"
   },
   "dependencies": {
-    "async": "^3.1.0",
     "chalk": "^2.4.2",
     "datauri": "^2.0.0",
     "htmlparser2": "^4.0.0",

src/css.js

@@ -2,7 +2,6 @@
 
 var url = require( "url" );
 var xtend = require( "xtend" );
-var parallel = require( "async" ).parallel;
 var path = require( "path" );
 var inline = require( "./util" );
 
@@ -91,8 +90,27 @@ module.exports = function( options, callback )
         index = found.index + index + 1;
     }
 
-    parallel( tasks, function( err )
+    var promises = tasks.map( function( fn )
     {
-        callback( err, result );
+        return new Promise( function( resolve, reject )
+        {
+            fn( function( error )
+            {
+                if ( error ) {
+                    reject ( error );
+                } else {
+                    resolve();
+                }
+            } );
+        } );
     } );
+
+    Promise.all( promises )
+        .then( function()
+        {
+            callback( null, result );
+        }, function( error )
+        {
+            callback( error, result );
+        } );
 };

src/html.js

@@ -3,7 +3,6 @@
 var path = require( "path" );
 var unescape = require( "lodash.unescape" );
 var xtend = require( "xtend" );
-var parallel = require( "async" ).parallel;
 var inline = require( "./util" );
 var css = require( "./css" );
 var htmlparser = require( "htmlparser2" );
@@ -262,8 +261,27 @@ module.exports = function( options, callback )
 
     result = replaceInlineAttribute( result );
 
-    parallel( tasks, function( err )
+    var promises = tasks.map( function( fn )
     {
-        callback( err, result );
+        return new Promise( function( resolve, reject )
+        {
+            fn( function( error )
+            {
+                if ( error ) {
+                    reject ( error );
+                } else {
+                    resolve();
+                }
+            } );
+        } );
     } );
+
+    Promise.all( promises )
+        .then( function()
+        {
+            callback( null, result );
+        }, function( error )
+        {
+            callback( error, result );
+        } );
 };