feat(options): add failOnUnused to enable generating error

* Original commit: 33d7a62
* Original author: @oliviertassinari
* Closes #3

lib/index.js

@@ -8,6 +8,7 @@ var glob = require("glob");
 function UnusedFilesWebpackPlugin (_options_) {
   var options = this.options = _options_ || {};
   options.pattern = options.pattern || "**/*.*";
+  options.failOnUnused = options.failOnUnused === true;
 
   var globOptions = this.globOptions = objectAssign({
     ignore: "node_modules/**/*"
@@ -22,7 +23,7 @@ UnusedFilesWebpackPlugin.prototype.apply = function(compiler) {
 UnusedFilesWebpackPlugin.prototype._applyAfterEmit = function(compiler, compilation, done) {
   var globOptions = this._getGlobOptions(compiler);
 
-  glob(this.options.pattern, globOptions, onGlobResult);
+  glob(this.options.pattern, globOptions, onGlobResult.bind(this));
 
   var fileDepsMap = this._getFileDepsMap(compilation);
 
@@ -37,8 +38,13 @@ UnusedFilesWebpackPlugin.prototype._applyAfterEmit = function(compiler, compilat
       });
       if (unused.length) {
         var errmsg = "UnusedFilesWebpackPlugin found some unused files:\n" + unused.join("\n");
+        var error = new Error(errmsg);
 
-        compilation.warnings.push(new Error(errmsg));
+        if (this.options.failOnUnused) {
+          compilation.errors.push(error);
+        } else {
+          compilation.warnings.push(error);
+        }
       }
     }
     done();