fix(bin/_mocha): Make --watch-extensions default to 'js'

The change in c580294 to `--watch-extensions` failed to set a default value. This change specifies
that default value.

Fixes mochajs#3336

bin/_mocha

@@ -201,7 +201,7 @@ program
   .option('--trace-deprecation', 'show stack traces on deprecations')
   .option('--trace-warnings', 'show stack traces on node process warnings')
   .option('--use_strict', 'enforce strict mode')
-  .option('--watch-extensions <ext>,...', 'specify extensions to monitor with --watch', list, [])
+  .option('--watch-extensions <ext>,...', 'specify extensions to monitor with --watch', list, ['js'])
   .option('--delay', 'wait for async suite definition')
   .option('--allow-uncaught', 'enable uncaught errors to propagate')
   .option('--forbid-only', 'causes test marked with only to fail the suite')