Add a terraform linter

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

doc/ale-hcl.txt

@@ -5,7 +5,7 @@ ALE HCL Integration                                           *ale-hcl-options*
 ===============================================================================
 terraform-fmt                                           *ale-hcl-terraform-fmt*
 
-See |ale-terraform-fmt| for information about the available options.
+See |ale-terraform-fmt-fixer| for information about the available options.
 
 ===============================================================================
   vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl: