Add support for custom wrapper for opening files on Linux.
Changed environment variable
Made the method simpler
Windows version of wrapper option
SHOTGUN_PLUGIN_LAUNCHER implementation for OSX.
Updated with detauls on SHOTGUN_PLUGIN_LAUNCHER.
Merge pull request #2 from lithorus/lithbrowser-plugin/master

Add support for custom wrapper for opening files via SHOTGUN_PLUGIN_LAUNCHER env var.