replace autotune element with x-autotune custom element.

[Kilian]

Here's the PR for the suggestion in #55. This makes Autotune use a custom element, making it more compliant with the HTML specification.

[Kilian]

Both jest and tslint are happy with the above changes. (You can set up something like Travis to run these automatically for every PR, by the way)

[dvdsgl]

This breaks production websites. Maybe we should add support for compliant alternatives but not remove the previous tag and attributes?

[Kilian]

There's a few options

1. Semantic versioning. Bump the major version because it's a breaking change.
2. Bump the major version, keep the <autotune>support as well, and add a deprecation message when used. Remove it in the next major version.
3. Add a function that checks for <autotune> elements, changes them all to <x-autotune> elements and then run the rest of the code.

This is also roughly in order of complexity :)

You already had support for both the autotune and the data-* attributes, so that's not changing other than me updating the documentation and samples to show just the compliant method.

[Kilian]

@dvdsgl any further thoughts on this?

[dvdsgl]

Sorry for the delay! Added a comment.

[Kilian]

Sorry for the even bigger delay! I've re-added support for <autotune> and added a test for it as well.