Add prettier, and related eslint, packages and config

Note: eslint-plugin-prettier is pegged to 2.6.0 because versions after this
expose a bug within ember-cli-eslint in which it breaks.

See ember-cli/ember-cli-eslint#235

Author: chadian

.eslintrc.js

@@ -5,11 +5,13 @@ module.exports = {
     sourceType: 'module'
   },
   plugins: [
-    'ember'
+    'ember',
+    'prettier'
   ],
   extends: [
     'eslint:recommended',
-    'plugin:ember/recommended'
+    'plugin:ember/recommended',
+    'plugin:prettier/recommended'
   ],
   env: {
     browser: true
@@ -21,6 +23,7 @@ module.exports = {
     {
       files: [
         '.eslintrc.js',
+        '.prettierrc.js',
         '.template-lintrc.js',
         'ember-cli-build.js',
         'index.js',

.prettierrc

@@ -0,0 +1,8 @@
+{
+  "printWidth": 100,
+  "semi": true,
+  "singleQuote": true,
+  "tabWidth": 2,
+  "trailingComma": "none",
+  "useTabs": false
+}