Support other paths for display plugins - environment variable

[ghost]

Currently, gol only looks in one path for plugins:

** (gol:19783): CRITICAL **: Display plugin directory isn't found: /usr/local/lib/growl-for-linux/display

What I would like to be able to do is EXPORT GOL_PLUGINS=~/gol/plugins and have it used there - this will make it easier for per-user installations and also make it possible to have gol included in Debian :P