Identify custom format

Usage:
```javascript
gm('./examples/imgs/original.jpg').format({ custom: '%f: %m, %wx%h' }, function (err, meta) {
	console.log(meta);
});

log:
original.jpg: JPEG, 460x155
```

It's very useful for get only specified fields of metadata (custom string format, delimiters, etc.) without verbose "slow" option.