Support custom style factory

[sgratzl]

closes #97 by allowing creating of alternative style tags. This can also be used to e.g don't have an inline style tag but manage it within the document body.

[sgratzl]

open issue: how to handle it with the bundle / vue to support that

[sgratzl]

open issue: how to handle it with the bundle / vue to support that

created an createElement function within the bundle that is wrapping Preact's createElement. Thus, you can write createElement the old way:

import {createElement} from "@upsetjs/bundle";

function customStyleFactory(rules: string) {
  return createElement(
    'style',
    {
      extra: 'abc',
    },
    rules
  );
}