Add quick-and-dirty way to control which subscription adapters are loaded

It's now possible to turn off Sunlight's default adapters. `config.yml.example` includes enough adapters to get the tests to pass. There should not be any change needed in production.

Need to do something about this line in `scout.rb` for the development environment:

```ruby
config.also_reload "./subscriptions/adapters/*.rb"
```

And need to do something about these lines in the `Rakefile` for cron jobs in production:

```ruby
subscription_types = Dir.glob('subscriptions/adapters/*.rb').map do |file|
  File.basename file, File.extname(file)
end
```

Any ideas?