Janitorial: add Prettier to help prettify JS code in Jetpack. (#10784)

#### Changes proposed in this Pull Request:

This PR adds support for Prettier to help prettify JS code in Jetpack.

The original configuration is copied from Calypso.

Down the road, I think we should, like in Calypso, add Prettier to our pre-commit hook to clean things up a bit.

#### Testing instructions:

* Configure Prettier in your editor.
* Run `yarn` 
* See that your editor can auto-fix your code 

#### Proposed changelog entry for your changes:
* none

.prettierrc

@@ -0,0 +1,12 @@
+useTabs: true
+tabWidth: 2
+printWidth: 100
+singleQuote: true
+trailingComma: es5
+bracketSpacing: true
+parenSpacing: true
+jsxBracketSameLine: false
+insertPragma: true
+requirePragma: false
+semi: true
+arrowParens: avoid

package.json

@@ -148,6 +148,7 @@
     "mockery": "1.7.0",
     "nock": "7.7.3",
     "node-wp-i18n": "1.2.1",
+    "prettier": "github:Automattic/wp-prettier",
     "react-test-renderer": "15.6.2",
     "sinon": "1.17.7",
     "sinon-chai": "2.14.0"