Added callback method: sanitize_callback

This callback enables change sanitize behavior.
Filename sanitization when sending to the device is not necessary for some devices.
Official plugin should never override this callback this callback to pass the
sanitization, but this is useful to override by the private driver plugin.

This is the another approach of #155 to respect Calibre's development policy.
I'm glad if this approach will be accpeptable for convenience of multibyte-region users.