[POC / WIP] ]Wait for an event before rendering the document

this shows a way on how we could wait for an event before rendering a pdf.

Note that I couldn't build what I did, as it seems some tools are missing on my env, hence only the poc, and maybe give some ideas ?

See [the mailing list](https://groups.google.com/forum/?hl=en%3Fhl%3Den#!topic/wkhtmltopdf-general/QBw6NIlP_PI)