[ShadyCSS] Expose prepareTemplate for non template DocumentFragments

[tomalec]

As briefly suggested at README.md#usage, to support VanillaJS elements I need to use ShadyCSS API manually.

1. First, call ShadyCSS.prepareTemplate(template, name) on a <template> element that will be imported into a shadowRoot

The problem is, I usually don't use a <template> to create shadowRoot (see https://github.com/webcomponents/shadycss/issues/144). Creating HTMLTemplateElement for every single string or DocumentFragment I want to put into my Shadow, seems like an overhead.

Could ShadyCSS expose a method to process DocumentFragment or string? Like StyleTransformer ;)