fix: Process all files sequentially

Fully asynchronous mode with some throttling, if there are many input
file, can be implemented later. Sequantial processing keeps the same
order of processed files; alphabetical sorting could be added, when the
asynchronous mode is introduced.

Author: prantlf

tasks/tidy-html5.js

@@ -45,7 +45,9 @@ module.exports = function (grunt) {
     }
 
     files.reduce(function (previous, file) {
-      return file.src.reduce(process, previous)
+      return previous.then(function () {
+        return file.src.reduce(process, Promise.resolve())
+      })
     }, Promise.resolve())
     .then(function () {
       const ok = failed ? force ? grunt.log.warn : grunt.fail.warn