Don't use ':' in backup directory names

':' is not allowed on FAT/exFAT filesystems, so backups to such
drives failed. Use '%Y-%m-%d_%H-%M-%S' instead; existing backups
with the old format still sort correctly. Document the limitations
of FAT/exFAT in the man page.

simple_backup/simple_backup.py

@@ -210,7 +210,9 @@ class Backup:
     # Function to create the actual backup directory
     def define_backup_dir(self) -> None:
         """Define the actual backup dir"""
-        now = datetime.now().strftime('%Y-%m-%d %H:%M:%S')
+        # Avoid ':', which is not allowed on FAT/exFAT filesystems.
+        # Backups created with the old format ('%Y-%m-%d %H:%M:%S') still sort correctly
+        now = datetime.now().strftime('%Y-%m-%d_%H-%M-%S')
         self._output_dir = f'{self.output}/simple_backup/{now}'
 
         if self._remote: