Add option for customized built-in elements

BetterHTMLElement.js

@@ -46,6 +46,9 @@ export class BetterHTMLElement extends HTMLElement {
 			})
 		})
 		name = name.replace(/([a-z])([A-Z])/g, "$1-$2").toLowerCase()
+		if (this.extends)
+			customElements.define(name, this, { extends: this.extends })
+		else
 			customElements.define(name, this)
 		return name
 	}