tar: create and validate a manifest from a tar stream

This commit contains added features to go-mtree that allows
user to create an mtree spec with '-T' option when specifying
a tar archive. Users can also validate an mtree spec against
a tar archive with an mtree spec. Also for the test archive,
there is a mixture of files, and folders (empty & non-empty),
and symlinks (broken & unbroken).

Signed-off-by: Stephen Chung <schung@redhat.com>

parse.go

@@ -81,6 +81,7 @@ func ParseSpec(r io.Reader) (*DirectoryHierarchy, error) {
 				e.Type = RelativeType
 			}
 			e.Keywords = f[1:]
+			// TODO: gather keywords if using tar stream
 			e.Parent = creator.curDir
 			for i := range e.Keywords {
 				kv := KeyVal(e.Keywords[i])