Register a callback that will get called right after generators has
finished.
This is useful if users want to process generated files.
For example, can execute an autocorrect of RuboCop for generated files
as like following.
```ruby
config.generators.after_generate do |files|
system("bundle exec rubocop --auto-correct " + files.join(" "), exception: true)
end
```
This Thor-action isolates the logic whether to (over-)write migration and
what is shown to the user. It's modelled after Thor's CreateFile-action.
This solves the issue that removing a non-existing migration, tried to
remove the template-path (#13588).
Related issues: #12674
