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

test/command_callback/test_terraform_terraform_command_callback.vader

@@ -0,0 +1,9 @@
+" Based upon :help ale-development
+Before:
+  call ale#assert#SetUpLinterTest('terraform', 'terraform')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The default command should be correct):
+  AssertLinter 'terraform', ale#Escape('terraform') . ' fmt -no-color --check=true -'