Customize token flow OAuth expirations with a config lambda

At NationBuilder, we assign a custom expiration to access tokens derived from token-based OAuth. We do this because our authorization code tokens tend to have very high expirations, while our token-based access tokens have very short ones. This pull request solves this problem for us by enabling the user to define an expiration on each app. Since we placed this configuration option inside of a lambda, it could also dynamically customized to vary in different ways.

-David
I now we stub it in other specs, but I'd prefer we use:
```ruby
Doorkeeper.configure do
  custom_access_token_expires_in do |_oauth_client|
    custom_ttl
  end
end
```