add attachment symlinks option, rename binary option to attachment with export type value

README.md

@@ -30,8 +30,14 @@ snac2nex -s /path/to/snac/storage -t /path/to/nex -u user1 -u user2
   -u, --user <USER>
           Username to export
 
-  -b, --binary
-          Export binary files (attachments)
+  -a, --attachment <ATTACHMENT>
+          Include attachment files
+
+          Supported values:
+
+          * `c` (`copy`) - copy attachment files export
+          * `h` (`hard`) - create hard links
+          * `s` (`soft`) - create soft links (macos, linux, windows only)
 
   -r, --rotate <ROTATE>
           Keep running as the daemon, renew every `n` seconds