show usage when called without files in tty

currently just calling `colorguard` with no options hangs, becuase we listen for `data` events for piped info.
When `isTTY` is true, we are not being piped, and as a result nothing will ever happen ever. which sucks.

show I added some metadata, and now it looks like this

```
 $ colorguard
 Usage: colorguard --file [style.css]

 Options:
   --file       A CSS file
   --threshold  Threshold of allowable color difference
   --options    An optional JSON file containing all options (Overrides `--threshold`)
   --format     Output type to full json
```