"state list" command, starting plumbing CLI

[mitchellh]

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.

Main changes in this PR:

• 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 issues, or things that aren't done in this PR on purpose:

• Unit tests for terraform state list are on the way. Unit tests for the core changes are all there.

[jen20]

Just rebased this and opened a new PR as #5811

[ghost]

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