Handle non-web dynos

This PR lets us define which types of dynos to monitor and manage via a simple config line like this:
```ruby
Whacamole.configure("HEROKU APP WITH MULTIPLE DYNO TYPES") do |config|
  config.api_token = ENV['HEROKU_API_TOKEN']
  config.dynos = %w{web worker scheduled}
end
```
Also supports the `+=` syntax:
```ruby
config.dynos += %w{worker scheduled}
```

Default is to follow the current behaviour: watching only `web` dynos.