feat(config): add usePolling config

This config option is forwarded to chokidar. When set to `true` (default value is `false`), chokidar uses `fs.watch` instead of stat polling.

This seems to be finally working well on linux, however it does not work on Mac.
See paulmillr/chokidar#62

That's why I'm leaving it undocumented ;-)

lib/config.js

@@ -263,6 +263,7 @@ var Config = function() {
   this.colors = true;
   this.autoWatch = false;
   this.autoWatchBatchDelay = 250;
+  this.usePolling = false;
   this.reporters = ['progress'];
   this.singleRun = false;
   this.browsers = [];

lib/watcher.js

@@ -71,9 +71,10 @@ var getWatchedPatterns = function(patternObjects) {
   });
 };
 
-exports.watch = function(patterns, excludes, fileList) {
+exports.watch = function(patterns, excludes, fileList, usePolling) {
   var watchedPatterns = getWatchedPatterns(patterns);
   var options = {
+    usePolling: usePolling,
     ignorePermissionErrors: true,
     ignoreInitial: true,
     ignored: createIgnore(watchedPatterns, excludes)
@@ -96,4 +97,4 @@ exports.watch = function(patterns, excludes, fileList) {
   return chokidarWatcher;
 };
 
-exports.watch.$inject = ['config.files', 'config.exclude', 'fileList'];
+exports.watch.$inject = ['config.files', 'config.exclude', 'fileList', 'config.usePolling'];