User needs to capture exitCode for a SpringApplication prior to the context closing.

Currently the only feature boot offers to do this is in the `SpringApplication.getExitCode` method which is private and only used by the `SpringApplication.exit(..)` which closes the context.  
While the user can home roll their own `getExitCode()` method to capture the data prior to the context getting closed, it would be useful to add a `ExitCodeGenerators` class that supports this feature.   
