qtlaoder.js: Support adding Init() arguments.

This is useful for sending arguments to the application
at startup. QtLoader users can now add key/value
pairs to config.environment. QtLoader will forward
these as attributes on the embed element. Finally
the key/value pairs will end up as arguments to
pp::Instance::Init()

Change-Id: Ic7ca434e9c097cd624b25692c250daa46a257c2f
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>