build(config): respect browserslist config

Currently gulp-autoprefixer has forced settings via overrideBrowserslist, however it's not recommended and breaks best practices as it silently overrides user's settings. As user's settings in package.json for example are not respected there will be useless CSS properties for modern browsers, for example existing config includes even IE 10

tasks/config/tasks.js

@@ -1,9 +1,21 @@
 var
+  browserslist = require('browserslist'),
   console = require('better-console'),
   config  = require('./user'),
   release = require('./project/release')
 ;
 
+var defaultBrowsers = browserslist(browserslist.defaults)
+var userBrowsers = browserslist()
+var hasBrowserslistConfig = JSON.stringify(defaultBrowsers) !== JSON.stringify(userBrowsers)
+
+var overrideBrowserslist = hasBrowserslistConfig ? undefined : [
+  'last 2 versions',
+  '> 1%',
+  'opera 12.1',
+  'bb 10',
+  'android 4'
+]
 
 module.exports = {
 
@@ -118,13 +130,7 @@ module.exports = {
 
     /* What Browsers to Prefix */
     prefix: {
-      overrideBrowserslist: [
-        'last 2 versions',
-        '> 1%',
-        'opera 12.1',
-        'bb 10',
-        'android 4'
-      ]
+      overrideBrowserslist
     },
 
     /* File Renames */