Add support for the --dump-dom flag

[SartoRiccardo]

The --dump-dom flag would be very beneficial for processing as sometimes you need some other return values from chromium (e.g. pixel perfect coordinates of the generated images, something which javascript can do), by simply outputting them in an invisible element of the html page.
This could be done with that flag, however you would need to redirect stdout of the executed command to a file, which you can't do currently without tinkering with the module yourself.

[vgalin]

Thanks for making this suggestion, this is quite interesting.
Being able to redirect the browser's output is related to #55 and will be implemented at the same time.