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.

[coreyfarrell]

@boneskull you suggested just providing current user-space API in node.js core rather than implementing an event emitter in the process object. A potential issue is that the user-space API has 4 functions exposed, I'm not sure where these functions would belong. Also potentially confusing that they are named the same as EventEmitter functions but do not take the same arguments (eventName would be redundant in the user-space module).

https://github.com/cfware/spawn-manipulate-env#usage