There were a lot of elements previously, like `Em`, `H1`, and a lot more, that took a string as the first argument previously. This was weird when you wanted to mix elements to output html like `<strong><em>…</em></strong>` or `<h1>Something <em>something</em> something</h1>`. gomponents is not an HTML validator, so I want people to be able to use elements however they please, also without text content.
This also means that all elements now have the same API.
This makes it clearer that the helpers return a `Node` of any kind, and that the type is not important.
This also streamlines the API, as attribute helpers already return just `Node`.
This makes it easier to use dot-imports.
Also updated the readme and examples with new usage, and move the `Classes` helper into the `components` package.
Previously, elements of kind void and empty elements generally would be rendered auto-closing (with a final `/` character in the start tag), which is allowed sometimes but arguably wrong. See https://dev.w3.org/html5/spec-LC/syntax.html#end-tags
This created problems with for example `textarea` and `script`, which cannot be auto-closing, or the browser renders it wrong.
Also clarified in the docs that this library outputs HTML5.
Fixes#42.
