Add support for type inference

[faulkner]

Type is optional (https://www.terraform.io/docs/configuration/variables.html#type), so infer the type when possible.

This fixes panics for cases like:

variable "foo" {
  default = []
}

[metmajer]

Great stuff @faulkner, thanks for the contribution!

[metmajer]

@faulkner I wanted to give you heads up that that maintenance is continued at segmentio/terraform-docs!