[feature request] job development workflow support

Hi, I'm contemplating using nomad-pack to well.. package all the jobs of my application in one neat package.

Here's my dream workflow:

# A. Include new job:
- Write/Develop a vanilla nomad job, with variables and good old `${var.myvar}` or `var.myvar` variable references. Note that my favourite source of variable is currently environment variables ( i.e. `NOMAD_VAR_myvar`)
- Include the job without any modification into my existing pack.
- Repeat for other jobs.

# B. Modify a job:
- Extract the job definition from the pack
- Modify independently
- Re-pack the job.

I'd like to work this way to avoid having to redeploy the whole pack to my dev environment each time I want to work on a given job.

As far as I understand, the way nomad-pack manages variables makes the workflow A impossible. I would have to change my job so the variable it uses (and the template syntax to access them) is now nomad-pack specific.

I suppose workflow B could be supported by the render command, but then I would lose the variable definitions.

I guess supporting workflow A would require nomad-pack to:

- Allow variables to be the superset of all variables mentioned in individual jobs.
- Allow individual jobs to keep using the `${var.}` or `var.` syntax
- Allow variables to come from environment variables. (Not a strong requirement).

Does all of this make sense or am I missing something fundamental?