Support quiet mode

[atjn]

Hi :)
This package is great, it does everything I need, except for the fact that it pollutes my console with query strings every time I use it. If I can't make the queries go away, I'll have to use something else.

I propose this change, that would simply stop printing the queries if the -quiet flag is set on the query. There are many other ways this could be done, but this would solve my issue. Let me know if you want it to be implemented in another way, I'll see if I can do that instead.

I believe this change closes #9

[atjn]

I am just now noticing that you haven't acknowledged a single pull request ever, but that you have silently implemented all the features in the pull requests.
A quick word of advice: that's a great way to push people away. Not just because they get angry at you, but because they lose confidence in you.
I have moved my project to use imagemin instead.