Support for hooking spawn / spawnSync without patching.

[coreyfarrell]

By default nyc 15 will be using node-preload instead of spawn-wrap. v0.1.x was fine for nyc's specific use case but I do not believe it's generic enough to be part of node.js. In particular it did not support alteration of variables (prepending / appending an existing value) nor does it allow control over conditional variable propagation (setting a variable only if it doesn't already exist).

I've created process-on-spawn. I think this may be generic enough to serve any situation where the environment of child processes must be centrally manipulated/controlled. This new module does not have direct support for propagating variables to grand-child processes and it doesn't help with adding --require options to NODE_OPTIONS. I've altering node-preload to use process-on-spawn instead of directly patching spawn / spawnSync functions, so the two modules work together.

My hope is to work towards getting the functionality of process-on-spawn into node.js so nyc can eventually avoid patching spawn / spawnSync.

Ref nodejs/node#27344
CC @bcoe

Edit: I've renamed the module to process-on-spawn which provides an options object to listener functions.

[wesleytodd]

@Qard Where is the best place for us to engage on the proposal for unified hooks and this feature?

[Qard]

The plan is to continue the discussion in nodejs/diagnostics#401. I'll be getting back to working on this stuff full-time again in about a week. Until then, I'll be a bit less active for now. If you have any thoughts to share now though, I'm happy to discuss. :)