Fix ignored whitespaces and line breaks in fenced code

[yhatt]

It resolves #4.

As like as HTML, JSX converts line breaks to single whitespace. It would bring collapsed whitespaces / line breaks at around highlighted element of code block.

Luckily, we can deal it with using JS expression. This update will convert line breaks in the rendered HTML into JS expression before applying to htm.

In short, it means converting HTM:

htm`
  <pre>
    test({
      foo: 0;
      bar: 0;
    })
  </pre>
`

to

htm`
  <pre>${'\n'}
    test({${'\n'}
      foo: 0;${'\n'}
      bar: 0;${'\n'}
    })${'\n'}
  </pre>
`