chore(infrastructure): Add npm run lint:html for screenshot pages (#3502)

### What it does

* Adds `npm run lint:html` command
    - Runs against `test/screenshot/spec/**/*.html` files (excluding `index.html`)
* Adds root `.htmllintrc` config file
    - Unfortunately, [`htmllint`](https://www.npmjs.com/package/htmllint-cli) doesn't support disabling individual rules with inline comments.
    - However, you can disable _all_ rules with a comment block:
        ```html
        <a>Allowed tag</a>

        <!-- htmllint-disable -->
        <b>Banned tag</b>
        <!-- htmllint-enable -->

        <code>Allowed tag</code>
        ```
* Fixes linter errors in `button`, `card`, `drawer`, `radio`, `switch`, and `textfield` pages'
* Adds `<label>` elements to `radio` and `switch` pages:
    ![radio page screenshot](https://user-images.githubusercontent.com/409245/45004682-845cbb80-afa3-11e8-919b-22780e2c9930.png)
    ![switch page screenshot](https://user-images.githubusercontent.com/409245/45004662-537c8680-afa3-11e8-8146-e8cfe60a0180.png)

### How to test

```bash
npm run lint:html
```

Author: acdvorak

.htmllintrc

@@ -0,0 +1,37 @@
+{
+  "attr-bans": [],
+  "attr-name-ignore-regex": "^(viewBox)$",
+  "attr-name-style": "dash",
+  "attr-no-dup": true,
+  "attr-req-value": false,
+  "attr-validate": false,
+  "class-no-dup": true,
+  "class-style": "bem",
+  "doctype-first": true,
+  "doctype-html5": true,
+  "fig-req-figcaption": true,
+  "head-req-title": true,
+  "head-valid-content-model": true,
+  "href-style": "relative",
+  "html-req-lang": true,
+  "html-valid-content-model": true,
+  "id-class-style": "bem",
+  "id-no-dup": true,
+  "img-req-alt": true,
+  "img-req-src": true,
+  "indent-delta": false,
+  "indent-style": "spaces",
+  "indent-width": 2,
+  "indent-width-cont": true,
+  "input-radio-req-name": true,
+  "input-req-label": true,
+  "line-max-len": 200,
+  "line-max-len-ignore-regex": "<script|<link|<path|^\\s*<!--\\s*https?://|^\\s*https?://|^\\s*href=\"[^\"]+\"",
+  "line-no-trailing-whitespace": false,
+  "raw-ignore-regex": "<!--\\s*htmllint-disable\\s*-->[^]*?<!--\\s*htmllint-enable\\s*-->|<svg[^>]*>[^]*?</svg>",
+  "tag-bans": ["b"],
+  "tag-name-lowercase": true,
+  "tag-name-match": true,
+  "title-max-len": false,
+  "title-no-dup": true
+}