Convert constructor to a connectedCallback function

Building a header button in memory with a custom heading ("level")
causes a unexpected behaviour. Consider the following example:

```js
const headerButton = document.createElement('md-header') // The constructor is executed at this point.
headerButton.setAttribute('level', '1')                  // Has no effect since the style has already been set.
headerButton.textContent = 'h1'
document.body.appendChild(headerButton)
```

While we expect to have a header button that outputs a single hash
(`#`), we have a button that outputs three (`###`). The constructor sets
the styles based on the `level` attribute when the component is
initialised.

Changing the constructor to a `connectedCallback` function
ensures that the button styles aren't set until the button is connected
to the DOM.

Author: koddsson

src/index.ts

@@ -111,9 +111,7 @@ class MarkdownButtonElement extends HTMLElement {
 }
 
 class MarkdownHeaderButtonElement extends MarkdownButtonElement {
-  constructor() {
-    super()
-
+  connectedCallback() {
     const level = parseInt(this.getAttribute('level') || '3', 10)
     if (level < 1 || level > 6) {
       return