(Enhancement or missing doc): Reduce verbosity

[EHJ-52n]

First, thank you very much for this cool module. Except some minor issues, it does what I need.

One thing, I would like to be configurable is the log output during authentication:

* pam_usb v0.5.0
* Authentication request for user "username" (sudo)
* Device "device-name" is connected (good).
* Performing one time pad verification...
* Regenerating new pads...
* Access granted.
[sudo] password for username:

I would like to have an configuration switch to reduce the above output to:

* Performing one time pad verification...
* Regenerating new pads...
[sudo] password for username:

or

[sudo] password for username:

What do you think?