Allow extra exif tags in --sort and --rename

[dryice]

Sometimes multiple person are using multiple devices to record the same event (e.g, a family outing).

In a case like this, it is useful to separate the pictures into different folders according to devices.

With this patch, we could, e.g, set sort_name to '%Y/%m/%d/{{ exif:model }}', the result will be something like 2017/12/05/Nikon D7000/* and 2017/12/05/iPhone 6p/*, etc

The example doesn't resolve everything (e.g, multiple person could use multiple devices of the same type, say iPhone X), but this patch support any exif tag that's ExifTool supported, so people could get creative and have the potential to solve it with some other tags :)