Add 'quiet' command

### Description
The 'quiet' command lets you copy to the clipboard without echo. This can be useful if the item information is somewhat private. It will still echo an error if the item is not found.


### Tests
I wasn't able to find a test for this feature. Since nothing is echoed I can't grep on anything like the other tests. Tried grep '^$' but that didn't work. Seems to work on my system however. 


### Disclaimer
This is my first ruby code and honestly just wanted to do a little something to get used to the syntax and project setup. The code I added is pretty much a duplicate of the 'echo' function with little changes. Opening this pull request so you can give me a little feedback and merge if you think it can be useful.