Avoiding Duplicate

Am Currently aware of the following

- https://github.com/hashicorp/hcl/issues/81
- https://github.com/hashicorp/hcl/issues/188
- https://github.com/hashicorp/hcl/issues/42

I have a configuration file that is over `900` lines and realised its all duplication.

### Instead of writing

```hcl
provider "A" {
var1 = "X"
var2 = "Y"
}

provider "B" {
var1 = "X"
var2 = "Y"
}

provider "C" {
var1 = "X"
var2 = "Y"
var3 = "Z"
}

extra "D" {
var1 = "X"
var2 = "Y"
}
```
### It would be nice to be able to write it as follows

```hcl
provider "A" , "B", "C" ; extra "D" {
var1 = "X"
var2 = "Y"
}
provider "C" "var3" = "Z"
```

### Another possibility would be the ability to set variables using `$`

```hcl
$tmp {
var1 = "X"
var2 = "Y"
}

provider "A" = $tmp
provider "B" = $tmp
provider "C" {
$tmp
var3 = "Z"
}
extra "D" = $tmp
```

### Rational
Configuration sometimes contain duplicate values, Been able to concatenate values or set variables makes `HCL` a much more powerful configuration language.

Review the related HCL issues linked in the description before deciding on scope. Compare the proposed grouped blocks and `$` variables with the existing HCL syntax and define which duplication-reduction behavior should be supported; done means the chosen syntax and semantics are specified and implemented consistently.

Written by the indexing model from the issue text.