Allow for plugins and config using an init hook

[frankcarey]

Right now, bash -c <my cmd script> is run for each command. It would be useful to have either a more complex or at least alternative to using bash.

Examples:

• Allow for a "config" file where all the environment variables are loaded into your environment before any commands are run.
• Add the ability to add in plugins/middleware, say for docker.
• Be able to run commands where the bash shell is not available.

[frankcarey]

This now works and has some tests. I've been testing this out locally as well for a few projects and things seem to be working well too.