Replace JSX with tagged template literals

[brendancarney]

This provides a few benefits:

• Can use React and convenient slate values in the same file
• No transpilation required to create a slate value
• Users don't need to use slate-hyperscript directly

Here's a potential API. The naming will almost surely change as I don't like it right now.

const structure = {
  blocks: {
    paragraph: "paragraph"
  },
  marks: {
    bold: "bold"
  }
}  

const { editor, html } = SlateTest({ plugins: Code() }, { structure });

const value = html`
  <paragraph>
    Some sample <bold>text</bold>
  </paragraph>
`

A few open questions:

• Should this be completely outside of the SlateTest function? In other words, should it be a separate import?
• The html name could be confusing, but it does automatically get you syntax highlighting depending on your editor setup.

[brendancarney]

Added https://github.com/ConvertKit/slate-plugins/tree/master/packages/slate-htm to potentially make this happen.

[brendancarney]

After writing many tests for Slate code here and in other repositories, I'm convinced this is the way to go now.