You can now set a `processes` variable, and indicate that multiple processes should be run

@zeke pointed out that there's a lot of repeated code in the service.json when you need to run multiple copies of the same service:

https://github.com/npm/newww/pull/164/files

I think this is a nice approach to reducing the footprint of service.json. I'm on the fence as to whether we should also fallback to variables in package.json:

* on one hand, there are repeated stanzas in service.json (scripts, description).
* on the other hand, it's nice that a service.json has all the necessary information required to execute a service in it:
  * module name.
  * script to run.
  * environment variables.
* there's no rule that says a service.json needs to be in the same folder as the module it is executing, which could make things a bit confusing. Take npme, the service.json sits at the top level directory, and describes several modules that need to run: policy-follower, registry-frontdoor, etc, it would be hard to know here to look for each of the package.jsons to pull default values from.

With this change, starting up multiple processes for newww becomes a slick little service.json, which looks like this:

```json
{
  "newww": {
    "description": "Run multiple copies of the newww web-server."
    "processes": 4,
    "scripts": {
       "start": "node server.js"
     },
    "env": {
      "PORT": "808%i",
      "NODE_ENV": "production"
    }
  }
}
```

Does this get things closer to what you're looking for @zeke?