Allow Config File Inheritance

[ryanmrodriguez]

Addresses #1390

---

Allow setting an inherit_from key in 'shipit.yml' config files.

As of now this key expects a string. It is allowed to be chained:

# shipit_2.yml
inherit_from: shipit_1.yml

c: true
d: 123

# shipit_1.yml
inherit_from: shipit_base.yml

c: false

machine:
  environment:
    PUBLIC: true
    TEST: true

# shipit_base.yml

a: 123
b: test
c: 1


machine:
  environment:
    PUBLIC: false

Loading 'shipit_2.yml' results in:

{"a"=>123, "b"=>"test", "c"=>true, "machine"=>{"environment"=>{"PUBLIC"=>true, "TEST"=>true}}, "d"=>123}

Merge (deep) precedence = child overwrites parent (config file specified in inherit_from key).

[casperisfine]

Open question: does a non existing path should noop or result in an error?

The problem is kinda always the same though, we don't have a good way of reporting shipit.yml errors, so perhaps nooping is the reasonable thing to do.