Support for multi-user environment ($SUBLIME_DATA)

Today we can launch Sublime Text with --data to use a custom data dir location. It would be good to have the option to set for example $SUBLIME_DATA to a custom location to make this change "permanent".

This would make it easier to use Sublime Text in a multi-user environment (like a Citrix/VNC terminal service type of corporate environment) where only one central installation of Sublime is present and each user have their own data directory.