Feature: allow custom optional closed tags

In react a custom component can be notated like this: ``, in hyperx you have to notate is like this ``.

I would like to notate optionally closable, non-html tags that could or could not be closed at the end.

So I would register with hyperx these in opts, under `optionallySelfClosing: []`. If they are closed (i.e. ending with `/>`) then they are treated as closed tags. If they are not (i.e. ending in `>`) then they have children and are not yet closed.

Another way would be to have a `loose` setting in `opts`. Where any tag, html or otherwise could be optional open or closed. However, this seems like more work to implement.