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-terraform.txt

@@ -3,7 +3,7 @@ ALE Terraform Integration                               *ale-terraform-options*
 
 
 ===============================================================================
-fmt                                                         *ale-terraform-fmt*
+terraform-fmt-fixer                                   *ale-terraform-fmt-fixer*
 
 g:ale_terraform_fmt_executable                 *g:ale_terraform_fmt_executable*
                                                *b:ale_terraform_fmt_executable*
@@ -20,6 +20,18 @@ g:ale_terraform_fmt_options                       *g:ale_terraform_fmt_options*
   Default: `''`
 
 
+===============================================================================
+terraform                                             *ale-terraform-terraform*
+
+g:ale_terraform_terraform_executable     *g:ale_terraform_terraform_executable*
+                                         *b:ale_terraform_terraform_executable*
+
+  Type: |String|
+  Default: `'terraform'`
+
+  This variable can be changed to use a different executable for terraform.
+
+
 ===============================================================================
 tflint                                                   *ale-terraform-tflint*