Change log level at runtime

* Added toggle_log_level to CLILogging which toggles the log level between debug and its original value
* Allow use_log_level_option to accept an options hash and install a signal handler which calls toggle_log_level upon receing the given signal

This implements #30
It can be private. But testing the toggling without access to this function would require a few more Process.kill()-calls with an one second sleep afterwards. I thought this would slow down the tests too much.