feat(yamllint): include for this repo and apply rules throughout

* Semi-automated using `ssf-formula` (v0.5.0)
* Fix errors shown below:

```bash
sudoers-formula$ $(grep "\- yamllint" .travis.yml | sed -e "s:^\s\+-\s\(.*\):\1:")
pillar.example
  6:23      warning  truthy value should be one of [false, true]  (truthy)

test/salt/pillar/kitchen.sls
  3:1       warning  missing document start "---"  (document-start)
  7:1       error    too many blank lines (1 > 0)  (empty-lines)

test/salt/pillar/default.sls
  1:1       warning  missing document start "---"  (document-start)
  3:23      warning  truthy value should be one of [false, true]  (truthy)
```

.yamllint

@@ -0,0 +1,16 @@
+# -*- coding: utf-8 -*-
+# vim: ft=yaml
+---
+# Extend the `default` configuration provided by `yamllint`
+extends: default
+
+# Files to ignore completely
+# 1. All YAML files under directory `node_modules/`, introduced during the Travis run
+ignore: |
+  node_modules/
+
+rules:
+  line-length:
+    # Increase from default of `80`
+    # Based on https://github.com/PyCQA/flake8-bugbear#opinionated-warnings (`B950`)
+    max: 88