Option to keep some destination files on generation

[Liooo]

When customizing generated models following the doc, it'd be nice if the user-added custom models resided in the same packages as the jet model, so that we don't have to discriminate the import path depending on custom or not. But currently the generator deletes the whole destination directory, so any files added under model/* are deleted.

What do you think of adding an option to keep some of the files under destination directory, for example:

• an option that receives a list of regex or Match for filepath.Glob of the files to keep (or delete)
• a boolean option to completely turn off the deletion (then let users delete whatever they want before the generation)