Adds support for --virtualenv when using supervisord

One issue we've had with the export is that our  setups normally run in a virtualenv environment. We'd like to avoid baking these into Procfile as we feel this isn't compatible with the original Heroku ethos with the file, so instead we've added an option to export --virtualenv=/path/to/virtualenv/activate. 

It works for our use case, but it might need to be tested for other use cases. Alternatively perhaps it could be generalized into a general "Run this command before running the Procline" (For whatever java/ruby/etc use to setup their environment.