Use JSON to support control characters

When exiftool returns tag fields on the command line it [converts control characters](http://owl.phy.queensu.ca/~phil/exiftool/faq.html#Q21) into `.`

You'll be familiar with this if you try to import images that have CR (char(13)) in the Caption-Abstract. 

This commit optionally supports the miniexif `-j` option via `use_json: true`. It will then use JSON to allow control characters in field values.

I'm happy to be told this isn't the way to do it - I just needed a solution to process text fields correctly - I can't be the only one.