Procedure::run should allow custom export variables

[andrewfenn]

Using Ubuntu linux as an example you may find yourself in need of defining custom bash variables before running the PhantomJS command.

The specific issue I ran into was that I needed to define the following..

# define how QT renders in PhantomJS
export QT_QPA_PLATFORM=offscreen
# define where fonts are loaded from (fixes issues rendering chinese fonts in screenshots)
export QT_QPA_FONTDIR=/usr/share/fonts

Currently what I've done to get around this is to simply change Procedure::run, proc_open call like so...

$process = proc_open('export QT_QPA_PLATFORM=offscreen; export QT_QPA_FONTDIR=/usr/share/fonts;'.escapeshellcmd(sprintf('%s %s', $this->engine->getCommand(), $executable)), $descriptorspec, $pipes, null, null);

As you can see I simply prepended them on a string. I think it would be good in the future if there was an interface for being able to define this without having to custom hack the code though.