Added Struct Support

[Circuit8]

Hey Matthew,

So I've added struct support. Turns out we didn't need to do any major change to the way we read from Exiftool, just adding the -struct argument seemed to do the trick.

There's a couple of things to bear in mind:

1. I manually updated Tags.ts to include a struct type definition for the one I was interested in. I'm not sure how to go about having this automatically parsed like you do in mktags, perhaps you could shed some light on that? I'm not sure how often you regenerate that file, is putting it in manually an option for now?
2. Currently the struct writing is a basic implementation that only writes strings, one level deep. From what I can tell this will cover the majority of use cases, but from what I understand Exiftool actually supports deeply nested structs, so might be something we could think about supporting in the future.

Let me know if there's any problems with this or if I've done some bad Typescript again!

[mceachen]

Wow, I think all 17 of codeclimate's errors were bogus, and I turned off codacy as well

[Circuit8]

Cool I've made those changes let me know what you think

[mceachen]

The windows CI builds are being flaky. I kicked off a rebuild of your commit.

[mceachen]

Thanks for the PR! Your code is now in v8.0.0.