Add Prettier for formatting (#5)

.prettierrc.json

@@ -0,0 +1,7 @@
+{
+  "trailingComma": "es5",
+  "tabWidth": 2,
+  "singleQuote": true,
+  "bracketSpacing": false,
+  "arrowParens": "always"
+}

README.md

@@ -72,6 +72,12 @@ To lint the project run:
 npm run lint
 ```
 
+## Formatting
+
+[Prettier](https://prettier.io/) is used for code formatting. It has been pre-configured according to the Polymer Project's style. You can change this in `.prettierrc.json`.
+
+Prettier has not been configured to run when commiting files, but this can be added with Husky and and `pretty-quick`. See the [prettier.io](https://prettier.io/) site for instructions.
+
 ## More information
 
 See [Get started](https://lit-element.polymer-project.org/guide/start) on the LitElement site for more information.

package-lock.json

@@ -5462,6 +5462,12 @@
       "integrity": "sha1-IZMqVJ9eUv/ZqCf1cOBL5iqX2lQ=",
       "dev": true
     },
+    "prettier": {
+      "version": "1.19.1",
+      "resolved": "https://registry.npmjs.org/prettier/-/prettier-1.19.1.tgz",
+      "integrity": "sha512-s7PoyDv/II1ObgQunCbB9PdLmUcBZcnWOcxDh7O0N/UwDEsHyqkW+Qh28jW+mVuCdx7gLB0BotYI1Y6uI9iyew==",
+      "dev": true
+    },
     "private": {
       "version": "0.1.8",
       "resolved": "https://registry.npmjs.org/private/-/private-0.1.8.tgz",

package.json

@@ -9,6 +9,7 @@
     "build": "tsc",
     "build:watch": "tsc --watch",
     "lint": "lit-analyzer && eslint 'src/**/*.ts'",
+    "format": "prettier src/* --write",
     "serve": "es-dev-server --app-index demo/index.html --node-resolve --watch --open",
     "test": "karma start karma.conf.cjs",
     "test:watch": "karma start karma.conf.cjs --auto-watch=true --single-run=false",
@@ -41,6 +42,7 @@
     "karma-mocha": "^1.3.0",
     "lit-analyzer": "^1.1.9",
     "mocha": "^6.2.2",
+    "prettier": "^1.19.1",
     "typescript": "^3.7.2"
   }
 }

src/my-element.ts

@@ -12,11 +12,10 @@
  * http://polymer.github.io/PATENTS.txt
  */
 
-import { LitElement, html, customElement, property, css } from 'lit-element';
+import {LitElement, html, customElement, property, css} from 'lit-element';
 
 @customElement('my-element')
 export class MyElement extends LitElement {
-
   static styles = css`
     :host {
       display: block;