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
I'd opt for removing the hash and go for a single argument called change_at_runtime with 'USR1' as the default. Passing false would disable the signal handler.