Makes the timeout option optional

The `--timeout` / `-t` option was made mandatory (by code) when introduced.
Omitting the option would cause a `KeyError: 'timeout`.

This commit makes it optional by adding a default value (of 60) when option is not given.

imapbackup.py

@@ -536,6 +536,8 @@ def get_config():
       config['port'] = 993
     else:
       config['port'] = 143
+  if not 'timeout' in config:
+    config['timeout'] = 60
  
   # done!
   return config