Enable Mergify support (#1059)

Related to https://github.com/dhall-lang/dhall-haskell/issues/1058

This is an attempt to test drive the Mergify GitHub app.  Unfortunately, the
only way to know for sure if the app works is to merge this configuration
into `master`, but if it does not work out then I will follow up with a pull
request to revert this change.

.mergify.yml

@@ -0,0 +1,12 @@
+pull_request_rules:
+- actions:
+    merge:
+      strict: true
+      method: squash
+  name: Automatically merge pull requests
+  conditions:
+  - status-success=continuous-integration/appveyor/pr
+  - status-success=hydra
+  - label=merge me
+  - ! '#approved-reviews-by>=1'
+  - ! '#changes-requested-reviews-by=0'