Parallelize execution directly via `critical`

[Tom-Bonnike]

This plugin currently runs on multiple files sequentially, which can potentially be very slow. critical is apparently looking at allowing multiple sources to be passed, we should use that when it’ll be possible:

• Multiple HTML sources addyosmani/critical#111
• Document concurrency option addyosmani/critical#371 (comment)
• How to generate critical css for lots of multiples pages serially? addyosmani/critical#320 (comment)
• How to get critical css from multiple external URLs? addyosmani/critical#215 (comment)

I tried running it in parallel with a promise pool but this led to out of memory errors (could maybe have been fixed by reducing the concurrency) and lingering Puppeteer processes (still TBD if that was the cause).