This replaces this plugin system with the extracted hashicorp/go-plugin
library.

This doesn't introduce any new features such as binary flattening but
opens us up to that a lot more easily and removes a lot of code from TF
in favor of the upstream lib.

This will introduce a protocol change that will cause all existing
plugins to have to be recompiled to work properly. There is no actual
API changes so they just have to recompile, but it is technically
backwards incompatible.

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 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.

I decided to split this up from the terraform state rm command to make the diff easier to see. These changes will also be used for terraform state mv.

This adds a `Remove` method to the `*terraform.State` struct. It takes a list of addresses and removes the items matching that list. This leverages the `StateFilter` committed last week to make the view of the world consistent across address lookups.

There is a lot of test duplication here with StateFilter, but in Terraform style: we like it that way.

This adds a field terraform_version to the state that represents the
Terraform version that wrote that state. If Terraform encounters a state
written by a future version, it will error. You must use at least the
version that wrote that state.

Internally we have fields to override this behavior (StateFutureAllowed),
but I chose not to expose them as CLI flags, since the user can just
modify the state directly. This is tricky, but should be tricky to
represent the horrible disaster that can happen by enabling it.

We didn't have to bump the state format version since the absense of the
field means it was written by version "0.0.0" which will always be
older. In effect though this change will always apply to version 2 of
the state since it appears in 0.7 which bumped the version for other
purposes.

… into the main terraform binary

This commit rectifies the fact that the original binary state is
referred to as V1 in the source code, but the first version of the JSON
state uses StateVersion: 1. We instead make the code refer to V0 as the
binary state, and V1 as the first version of JSON state.

This commit adds the groundwork for supporting module outputs of types
other than string. In order to do so, the state version is increased
from 1 to 2 (though the "public-facing" state version is actually as the
first state file was binary).

Tests are added to ensure that V2 (1) state is upgraded to V3 (2) state,
though no separate read path is required since the V2 JSON will
unmarshal correctly into the V3 structure.

Outputs in a ModuleState are now of type map[string]interface{}, and a
test covers round-tripping string, []string and map[string]string, which
should cover all of the types in question.

Type switches have been added where necessary to deal with the
interface{} value, but they currently default to panicking when the input
is not a string.

This changes the representation of maps in the interpolator from the
dotted flatmap form of a string variable named "var.variablename.key"
per map element to use native HIL maps instead.

This involves porting some of the interpolation functions in order to
keep the tests green, and adding support for map outputs.

There is one backwards incompatibility: as a result of an implementation
detail of maps, one could access an indexed map variable using the
syntax "${var.variablename.key}".

This is no longer possible - instead HIL native syntax -
"${var.variablename["key"]}" must be used. This was previously
documented, (though not heavily used) so it must be noted as a backward
compatibility issue for Terraform 0.7.