Add a terraform linter

[maxwell-k]

This linter uses the check functionality built into terraform. ALE
already has a fixer using terraform fmt but this doesn't provide error
messages. ALE already has a linter using tflint but this requires an
extra application to be installed.

For example this linter will give a warning that ! is an illegal
character in the line below:

variable "example" !{}

This linter runs the buffer through the command below and parses the
output:

terraform fmt -no-color -check=true -

This commit includes a basic implementation, documentation and tests.
The only option is to control which executable is run.

Tested with:

$ terraform -version
Terraform v0.11.13

[w0rp]

Cheers! 🍻