Add string diff support

[tmbb]

I might submit a PR handling this one of these days, if you're interested.

[olafura]

I'm definitely interested, it's just been a nice to have but for anybody that needs it I think they would be thankful.

I really need to write a contributor's guide, the most important thing at the moment is keeping good enough compatibility with jsondiffpatch. Good enough because sometimes there are better ways of doing some of the diffs that will still translate into the same outcomes when using the library for patching.

My most recent PR focused on lists and I switched to using a built in function for that. I know that most string diffs are similar to lists.

[tmbb]

String diffs are supported by jsonpatchdiff: https://github.com/benjamine/jsondiffpatch#live-demo, so I think we can support it too. I'm still not sure whether I actually need JSON diffs for my project, so I'm not sure I'll actually work on the PR.

[olafura]

@tmbb No problem, I'm not using this library for work either. I did have ideas of using it for my company I worked for because we were dealing with a really big and nasty json structure and json diff works really well with those. It's sort of like when dealing with legacy databases. You usually don't want to use these kinds of techniques unless you have to 😄

There might be a killer feature for this library, maybe I could look into doing it for the graphql queries we are doing, who knows.