Support deep merge

[dankelleher]

I would like to be able to do something like this:

package.json

{
  "dependencies": {
    "dep1": "1.0.0",
    "dep2": "1.0.0"
  }
}

environments.json

{
  "dev": {
    "dependencies": {
      "dep1": "1.2.0"
    }
  },
  "staging": {
    "dependencies": {
      "dep1": "1.1.0"
    }
  }
}

At the moment this is not possible, because the merge is shallow, so running penv dev with the above configuration would remove dep2.

[dankelleher]

PR here: #4

[julianduque]

@dankelleher landed in v0.2.0! Thanks!!! <3