Expression list indexing, slicing, and splatting.

[Paul-Ferrell]

Allow list indexing and slicing.

variables: 
  foo: 
    - mod1
    - mod2
    - mod3
  nums: [1, 2, 3, 4]
  bar: '{{foo[0] + foo[-1]}}'
  baz: '{{foo[1:]}}'

  build:
    modules:
      - 'other_mod1'
      - '{{baz}}'

bar would be equal to '3'
baz would be a list of ['mod2', 'mod3'], and 'build.modules' would be ['other_mod1', 'mod2', 'mod3']

The changes is actually fairly simple; when resolving strings, allow lists of strings instead of just strings. When these are returned, if the resolved string is already part of a list, extend the whole list. It will be an error to extend a list of other structures, however, or get back anything more complex than a list of strings.