Feature Request: Expose method processCss which returns inlined css only

[Warxcell]

Use-case is to be able to use critters inside SSR, by processing only CSS and caching processed CSS - allowing the content to evolve independently from the processed CSS. Something like

let html = .... rendered html in ssr ...
const cacheKey = .... some logic ....
const criticalCSS = getInlinedCriticalCSSFromCache(cacheKey) ?? critters.processCss(html);

html = html.replace('</head>', criticalCSS + '</head>')

This will help also with #78