Add filterable frontend indicator method

Resolves #507 

![screen shot 2014-05-06 at 3 37 29 pm](https://cloud.githubusercontent.com/assets/522158/2895319/4cc2cdc8-d55e-11e3-9224-6b83f19a7d2c.png)

Can be easily removed, if desired, like so:

```php
add_filter( 'wp_stream_frontend_indicator', function() { return false; } );
```

@lukecarbis Please review