prune: add --keep-younger-than=DATE

The format used for DATE is "%Y-%m-%d %H:%M:%S %z"

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

doc/ostree-prune.xml

@@ -89,6 +89,14 @@ Boston, MA 02111-1307, USA.
                 </para></listitem>
             </varlistentry>
 
+            <varlistentry>
+                <term><option>--keep-younger-than</option>=DATE</term>
+
+                <listitem><para>
+                    All commits older than DATE will be pruned.
+                </para></listitem>
+            </varlistentry>
+
             <varlistentry>
                 <term><option>--depth</option>=DEPTH</term>