time: add time_ago function from Damus iOS

Useful relative time formatting

Signed-off-by: William Casarin <jb55@jb55.com>

src/lib.rs

@@ -10,6 +10,7 @@ mod result;
 mod filter;
 mod ui;
 mod timecache;
+mod time;
 mod frame_history;
 mod timeline;