HTMLDocument.registerElement is deprecated.

Use `customElements.define` instead of `document.registerElement` see https://javascript.info/custom-elements
```
HTMLDocument.registerElement (file:///Users/user/Desktop/Atom.app/Contents/Resources/app.asar/static/index.js:160:12)
f (/Users/user/.atom/packages/script/node_modules/atom-space-pen-views-plus/lib/main.js:26:87233)
e.openTag (/Users/user/.atom/packages/script/node_modules/atom-space-pen-views-plus/lib/main.js:26:91114)
e.tag (/Users/user/.atom/packages/script/node_modules/atom-space-pen-views-plus/lib/main.js:26:90760)
Function.div (/Users/user/.atom/packages/script/node_modules/atom-space-pen-views-plus/lib/main.js:26:88521)
Function.content (/Users/user/.atom/packages/script/lib/header-view.js:48:19)
```

Start by tracing the call from lib/header-view.js:48 into atom-space-pen-views-plus/lib/main.js and the deprecated document.registerElement shown in the stack trace. Determine where the replacement belongs and verify that the warning no longer occurs when the header view is opened.

Written by the indexing model from the issue text.