Fixes #8: if unknown APKs found, prompt to use -c or use --delete-unknown

This adds the option --delete-unknown for the current default behavior of
`fdroid update`: to delete any unknown APKs.  Instead, it just outputs a
warning about the unknown APKs and suggests -c for adding it.

Fixes #8 https://gitlab.com/fdroid/fdroidserver/issues/8

completion/bash-completion

@@ -129,7 +129,7 @@ __complete_install() {
 __complete_update() {
 	opts="-h -c -v -q -b -i -I -e -w"
 	lopts="--help --createmeta --verbose --quiet --buildreport --interactive
- --icons --editor --wiki --pretty --clean"
+ --icons --editor --wiki --pretty --clean --delete-unknown"
 	case "${prev}" in
 		-e|--editor)
 			_filedir