Allow to specify a custom path for Chromium

[ggrossetie]

Asciidoctor Web PDF should support PUPPETEER_EXECUTABLE_PATH environment variable.
Please note that Puppeteer strongly recommends to use the exact same version so there will be not guarantee but it could still be useful in some cases.

Here's what you can read on the Puppeteer documentation:

We see Puppeteer as an indivisible entity with Chromium. Each version of Puppeteer bundles a specific version of Chromium – the only version it is guaranteed to work with.
This is not an artificial constraint: A lot of work on Puppeteer is actually taking place in the Chromium repository. Here’s a typical story (...)
However, oftentimes it is desirable to use Puppeteer with the official Google Chrome rather than Chromium. For this to work, you should install a puppeteer-core version that corresponds to the Chrome version.

https://github.com/puppeteer/puppeteer#q-why-doesnt-puppeteer-vxxx-work-with-chromium-vyyy