Add `align` to `<div>`s.

This gives us our first glimpse of reflecting attributes as properties.
Use default export for `CustomHTMLDivElement`.
Add `reflectAttr` helper.

This should DRY up future such properties.
Add .editorconfig file.
Merge pull request #13 from dglazkov/div-align

Add align attribute to divs, and some cleanups on top of this