Disable by Regular Expression

A feature request, not a bug.

I'd like to be able to disable `bot-away` by namespace. So in Refinery CMS say, I don't want `bot-away` to run on any non-public facing admin controllers. This is how I expect it would work:

``` ruby
BotAway.disabled_for controller: /^Admin/
```

But right now I have to specifically exclude each controller.

Thanks for a great gem by the way!
