Add in ability to save output report to a file.

[chappy84]

Particularly useful with json output.

I've been running this with the a local audit-db, however there doesn't seem to be a flag to silence the progress of reading this local DB.
I was sending the output of the command as a whole to a file using >, however that meant the above mentioned progress was also sent to the file (obviously)
e.g.

   0/990 [>---------------------------]   0%
  99/990 [==>-------------------------]  10%
 198/990 [=====>----------------------]  20%
 297/990 [========>-------------------]  30%
 396/990 [===========>----------------]  40%
 495/990 [==============>-------------]  50%
 594/990 [================>-----------]  60%
 693/990 [===================>--------]  70%
 792/990 [======================>-----]  80%
 891/990 [=========================>--]  90%
 990/990 [============================] 100%
{
    "guzzlehttp\/guzzle": {
        "version": "5.3.4",
        "advisories": {
            "guzzlehttp\/guzzle\/CVE-2016-5385.yaml": {
                "title": "HTTP Proxy header vulnerability",
                "link": "https:\/\/github.com\/guzzle\/guzzle\/releases\/tag\/6.2.1",
                "cve": "CVE-2016-5385"
            }
        }
    }
}

The composer --quiet flag stops ALL output, including the json report, so I thought it might be an idea to allow the report to be output to a file. The code changes in this PR do this.

[chappy84]

@sbuzonas any chance you could have a look over this?