Add a preStart hook

This would be similar to postBuild, but would be run at runtime, before execution of the actual notebook. This easily allows things such as:

1. setting environment variables at runtime
2. Minor adjustments that can’t be performed at build time
3. New things we haven’t thought of yet!
4. Custom parameters for notebooks / other applications

To prevent people from putting too many things here, it should have a default short timeout (5s?) to start.

The requirements for this script should be:

1. Be marked executable, can be in any language
2. Receives as parameters arguments to exec that it must perform as a final line to actually start the process we want (we will provide templates for this in various languages)
3. Not run for more than x seconds.

[choldgraf]

ping on this issue as a few people brought up a use-case @ the binder/jupyterhub talk here in Paris today:

many folks want to decouple the “content” of the repository from the “environment” needed to run that content (e.g. they want to be able to have notebooks live online independently of a Binder repo, and use the repo just to define the environment needed. Then they could update notebooks without needing to re-build the Binder image each time. This could be accomplished with a script such as this one by using gitpull or something like this to pull in content after the server starts.

[choldgraf]

I think this PR is firmly in the territory of “the project is 👍 on seeing this happen, it’s just a question of somebody having the time to submit a PR!”