Added get_file_reports in VT API module and changed API call in lookup_h...

The VT API call to look up the hashes was using `domain/report` endpoint which only works for the domains.
Now `api.py` contains a new method to look up the hashes: `get_file_reports`. This method is used by `lookup_hashes.py` instead of `get_domain_reports`.

*Note:*
VT API endpoint `file/report` accepts `resource` parameter which can be a single hash or a list of hashes, in CSV format. We are not using that so far but it could speed up the requests by batching them.