This introduces the terraform state list command to list the resources
within a state. This is the first of many state management commands to
come into 0.7.

This is the first command of many to come that is considered a
"plumbing" command within Terraform (see "plumbing vs porcelain":
http://git.661346.n2.nabble.com/what-are-plumbing-and-porcelain-td2190639.html).
As such, this PR also introduces a bunch of groundwork to support
plumbing commands.

The main changes:

- Main command output is changed to split "common" and "uncommon"
  commands.

- mitchellh/cli is updated to support nested subcommands, since
  terraform state list is a nested subcommand.

- terraform.StateFilter is introduced as a way in core to filter/search
  the state files. This is very basic currently but I expect to make it
  more advanced as time goes on.

- terraform state list command is introduced to list resources in a
  state. This can take a series of arguments to filter this down.

Known is...