Add option to keep specific stale files

[Michael-Ziluck]

There is the option for removeStaleFiles but it only allows you to keep them all or keep none. I'd like to have some custom files in the generated library but delete any other old ones.

It would be nice if there was an option called preserveCustomFile with a description of something like "When specified and removeStaleFiles is true, preserves the given files when other stale files are removed."

[luisfpg]

Out of scope, but a PR is welcome