Add preference EXCLUDE_GITIGNORE to exclude all files listed in .gitignore from being deleted if DELETE_FILES is set

I like the feature that files are deleted (DELETE_FILES = true). That makes sense for me in many use cases (e.g. uninstalling a WordPress plugin by deleting it in the repo). As you decribed in the documentation this should be used carefully and the EXCLUDE variable should be set to avoid deleting files by mistake. But I think there a lot of use cases in which the EXCLUDE variable and the .gitignore file have identical content. For this reason I added a new flag EXCLUDE_GITIGNORE to add all files and directory in .gitignore to the exclude list.