new option "--exclude-if-present" for bup-index

Excludes all directories that contain a file with the specified name.
So you can use "touch .do-not-backup-me" to exclude a directory from backup,
when you run bup index with "--exclude-if-present .do-not-backup-me"

Signed-off-by: Tim Riemenschneider <git@tim-riemenschneider.de>
new option "--exclude-caches" for bup-index

Excludes all directories that contain a file CACHEDIR.TAG that begins with
"Signature: 8a477f597d28d172789f06886806bc55".

See http://www.brynosaurus.com/cachedir/ for more information about
cache-directory tagging.

Signed-off-by: Tim Riemenschneider <git@tim-riemenschneider.de>