Allow custom loggers

[robinkrahl]

Currently, libnitrokey prints all log messages to stderr/std::clog. It would be useful to be able to specify a custom logging function instead to allow applications to use their own logging mechanisms.

As far as I see, there already is a NitrokeyManager::set_log_function method. This function could be added to the C API and adapated to accept the log level and the raw log message instead of the fully formatted log message.

[robinkrahl]

This would allow us to integrate the libnitrokey log messages with our own log messages in nitrocli, see d-e-s-o/nitrocli#177.