refactor gulpfile to work with gulp v4

gulpfile.js

@@ -1,43 +1,47 @@
 'use strict';
 
-var gulp = require('gulp'),
-    fs = require('fs'),
-    plumber = require('gulp-plumber');
+const gulp = require('gulp'),
+  fs = require('fs'),
+  plumber = require('gulp-plumber');
 
-var options = {
+const options = {
   source: 'templates',
   dist: 'dist',
   workingDir: 'tmp',
-  src: function plumbedSrc(){
-    return gulp
-      .src
-      .apply(gulp, arguments)
-      .pipe(plumber());
+  src: function plumbedSrc() {
+    return gulp.src.apply(gulp, arguments).pipe(plumber());
   }
 };
 
-/** Load tasks from the '/tasks' directory.
- * Look for .js & .coffee files.
- * Each file should correspond to a task.
+/**
+ * Load tasks from the '/tasks' directory.
  */
-fs
-  .readdirSync('./tasks')
-  .filter(function readJSFiles(file) {
-    return (/\.(js|coffee)$/i).test(file);
-  })
-  .map(function loadTasks(file) {
-    require('./tasks/' + file)(options);
-  });
+const build = require('./tasks/build')(options);
+const checkDeps = require('./tasks/check-deps')(options);
+const dupe = require('./tasks/dupe')(options);
+const less = require('./tasks/less')(options);
+const lint = require('./tasks/lint')(options);
+const postcss = require('./tasks/postcss')(options);
+const sass = require('./tasks/sass')(options);
 
-/** By default templates will be built into '/dist', then gulp will watch for changes in '/src'. */
+/* By default templates will be built into '/dist' */
 gulp.task(
-  'default', 
-  [
-    'dupe', 
-    'less',
-    'sass',
-    'postcss',
-    'lint',
-    'build'
-  ]
+  'default',
+  gulp.series(
+    ('dupe', 'less', 'sass', 'postcss', 'lint', 'build'),
+    () => {
+      /* gulp will watch for changes in '/templates'. */
+      gulp.watch(
+        [
+          options.source + '/**/*.html',
+          options.source + '/**/*.css',
+          options.source + '/**/*.scss',
+          options.source + '/**/*.less',
+          options.source + '/**/conf.json'
+        ],
+        { delay: 500 },
+        gulp.series('dupe', 'less', 'sass', 'postcss', 'lint', 'build')
+      )
+    }
+  )
 );