The option is named --exclude-if-present and accepts a parameter
filename[:content]. Directories are excluded and their contents is not
backed up if they contain a file with the specified name and,
optionally, that starts with the specified content. The tagfile itself
is never excluded.
There is also a shortcut --exclude-caches that works in the same way as
the likewise-named option of tar(1): Directories are recognized as cache
if they contain a file named "CACHEDIR.TAG.
Closes #317.
