Git-safe builds (ignore fnmatch strings when cleaning)

Sorry about the duplicate request. Something got corrupted in my version, then my attempts to fix it caused Github to auto-close the request.

This adds a `FREEZER_IGNORED_FILES` config which will cause build directory cleaning to skip any files which fnmatch a string in the ignored list. It works by removing matching entries from the set of extra files.

Since Frozen-Flask doesn't copy over non-generated files, no further modifications are necessary to make it git-safe. Also, I don't think that it should be changed to copy over favicon.ico and etc., in keeping with Flask's minimalism. This should be left up to the calling code.