Add config file handling

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Document configuration file

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Add keyring options

Add --keyring and --no-keyring to the command line options and keyring to
section [general] in the configuration file.

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>