Add Support for different docker-machine VM names

Add the variable BASH_IT_DOCKER_MACHINE to set a name other than 'dev' as
Virtual Machine name.

plugins/available/docker-machine.plugin.bash

@@ -1,11 +1,13 @@
 cite about-plugin
 about-plugin 'Helpers to get Docker setup correctly for docker-machine'
 
+[ -z ${BASH_IT_DOCKER_MACHINE+x} ] && BASH_IT_DOCKER_MACHINE='dev'
+
 # Note, this might need to be different if you use a machine other than 'dev'
 if [[ `uname -s` == "Darwin" ]]; then
   # check if dev machine is running
-  docker-machine ls | grep --quiet 'dev.*Running'
+  docker-machine ls | grep --quiet "$BASH_IT_DOCKER_MACHINE.*Running"
   if [[ "$?" = "0" ]]; then
-    eval "$(docker-machine env dev)"
+    eval "$(docker-machine env $BASH_IT_DOCKER_MACHINE)"
   fi
 fi