Add support for default values for variable types list and map.

[metmajer]

Default values are presented in JSON. The following example shows the processing of a Terraform variable of type map:

variable "foo" {
  default = {
    bar = 1
    baz = "two"
  }

  description = "Some variable named 'foo'."
  type = "map"
}

$ terraform-docs json .

[…]
{
  "Name": "foo",
  "Description": "Some variable named 'foo'.",
  "Default": {
    "Type": "map",
    "Value": {
      "bar": 1,
      "baz": "two"
    }
  },
  "Type": "map"
}
[…]

$ terraform-docs md .

Name	Description	Type	Default	Required
foo	Some variable named 'foo'.	map	{ "bar": 1, "baz": "two" }	no

$ terraform-docs .

[…]
var.foo ({ "bar": 1, "baz": "two" })
Some variable named 'foo'.
[…]