Added a `verbose` command line option.

In order to improve the signal to noise ratio of shifter,I added a `verbose` option to print `[info]` and `[queue]`
messages.

With this option, which defaults to false, those messages are no longer printed.

I felt those message were more useful to debug shifter itself than to those using shifter.
 
With less clutter on the report, the errors and warnings stand out much more.