levels filter added for query on file transport

query on file transport by levels

eg:

```Javascript
var winston = require('winston');
var logger = new (winston.Logger)({
	transports: [
	  new (winston.transports.File)({ filename: 'logfile.log' })
	]
});

logger.query({
  levels : ['info', 'warn']
},function (err, results) {
  if (err) {
    throw err;
  }
  console.log(results);
});

```