Custom Elements v1 support

[yuki24]

I'm sure you are aware of this, but I noticed that Trix uses the deprecated Custom Elements v0. I also noticed there's a branch for v1 @javan worked on. I rebased against the latest master and built the project to see if it works fine, and it seems to be working without problems and all the tests pass.

I understand that this is technically going to be a breaking change for folks who are polyfilling Custom Elements v0 by themselves to make Trix work, but is there any plan to merge the custom-elements-v1 into master?

[javan]

I'd definitely like to finish this branch and switch Trix to v1. I don't recall the exact details, but I ran into some issues with older browsers. v1 is trickier to polyfill (https://github.com/webcomponents/custom-elements#es5-vs-es2015) and that particular polyfill has a few issues. This one is a blocker for us: webcomponents/custom-elements#104.