Adding experimental pre/post deploy script support. If present, scripts in /home/git/predeploy.d and /home/git/postdeploy.d will be invoked before and after deploystep.

Also tweaked the deploystep script itself to receive one or more custom environment variables from /home/git/$APP/CUSTOM_ENV and inject them into the docker container. There's almost certainly a better way to get multiple custom environment vars into the container, though.