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.txt

@@ -2246,7 +2246,8 @@ documented in additional help files.
   tcl.....................................|ale-tcl-options|
     nagelfar..............................|ale-tcl-nagelfar|
   terraform...............................|ale-terraform-options|
-    fmt...................................|ale-terraform-fmt|
+    terraform-fmt-fixer...................|ale-terraform-fmt-fixer|
+    terraform.............................|ale-terraform-terraform|
     tflint................................|ale-terraform-tflint|
   tex.....................................|ale-tex-options|
     chktex................................|ale-tex-chktex|