build: A basic appveyor.yml

In the long run we intend to transition to using CircleCI for all of the
testing steps here, but we need to keep appveyor enabled here until we
have both the HCL 1 and HCL 2 branches ready to use CircleCI, so this is
just a minimal appveyor configuration for running our tests in a Windows
environment in order to avoid errant test failures on PRs until we have
completed the move fully to CircleCI.

appveyor.yml

@@ -0,0 +1,13 @@
+build: off
+
+clone_folder: c:\gopath\src\github.com\hashicorp\hcl
+
+environment:
+  GOPATH: c:\gopath
+  GO111MODULE: on
+  GOPROXY: https://goproxy.io
+
+stack: go 1.12
+
+test_script:
+  - go test ./...