chore(): add missing parameter to gulp task test

Gulp task `test` was missing the `--file` optional argument that could be passed in in the list of available options.

build/gulp/tasks/test.ts

@@ -35,6 +35,7 @@ export class GulpTask extends BaseGulpTask {
    */
   public static options: any = {
     'debug': 'Set karma log level to DEBUG',
+    'file': 'Changed file to filter tests down to just that file',
     'specs': 'Output all tests being run',
     'verbose': 'Output all TypeScript files being built & set karma log level to INFO',
     'watch': 'Adds Chrome browser and start listening on file changes for easier debugging'