add support for self-closing tags

[JRJurman]

Summary

This includes the changes to support self-closing tags (as documented in choojs/hyperx#58)

Resolves #36

Changes

• allow custom elements to be self-closing

const emptyTag = () => html`<span>my tag</span>`
const html = Tram.html({'empty-tag': emptyTag})

// before
html`<div><empty-tag></empty-tag>Hello</div>`
//  returns: <div><span>my tag</span>Hello</div>

// after
html`<div><empty-tag />Hello</div>`
// returns:  <div><span>my tag</span>Hello</div>

Warning

This may break current functionality depending on how you wrote tags that were already considered self-closing (e.g. <br>). Those tags need to now have an ending /> in their tag.

[ethanjurman]