This commit removes the ability to index into complex output types using
`terraform output a_list 1` (for example), and adds a `-json` flag to
the `terraform output` command, such that the output can be piped
through a post-processor such as jq or json. This removes the need to
allow arbitrary traversal of nested structures.
It also adds tests of human readable ("normal") output with nested lists
and maps, and of the new JSON output.
Seth Vargo