Add configuration option to skip splitting replies

@ygnessin 

For emails with large bodies, splitting the replies out can take an excessive amount of time. This new configuration option gives the app developer control over whether or not the replies are split out of the email body, the default behavior it to always split replies.

To skip splitting replies, add this to the app initializer:

```ruby
Griddler.configure do |config|
  config.split_replies = false
end
```