Add unpkg key to package.json

This provides a completely bundled distribution, ready importing from CDNs like unpkg.com for use in sandboxes like Codepen.  It allows this code to work:

```typescript
import { create as createJSS } from 'https://unpkg.com/jss';
```

The PR is a work-in-progress, but I'd rather start a conversation with an example than with prose.  As it is, I can't get `sizeSnapshot` to work with the bundled version.

I also wonder how best to handle plugins/presets.  The easiest options would be to add `unpkg` entry points to each `package`, and force authors to use multiple imports:

```typescript
import { create as createJSS } from 'https://unpkg.com/jss';
import createDefaultJSSPreset from 'https://unpkg.com/jss-preset-default';
```

I wonder what the best way to produce a batteries-included bundle is.  Perhaps there should be another bundle `jss-all` that exports all the other packages.  Then usage would be:

```typescript
import * as jssAll from 'https://unpkg.com/jss-all';

const jss = jssAll.create(jssAll.preset());
```

packages/jss-preset-default/package.json

@@ -6,6 +6,7 @@
   "homepage": "https://cssinjs.org/jss-preset-default",
   "main": "dist/jss-preset-default.cjs.js",
   "module": "dist/jss-preset-default.esm.js",
+  "unpkg": "dist/jss-preset-default.bundle.js",
   "typings": "./src/index.d.ts",
   "author": {
     "name": "Oleg Slobodskoi",

packages/jss/package.json

@@ -6,6 +6,7 @@
   "homepage": "https://cssinjs.org/",
   "main": "dist/jss.cjs.js",
   "module": "dist/jss.esm.js",
+  "unpkg": "dist/jss.bundle.js",
   "typings": "./src/index.d.ts",
   "author": {
     "name": "Oleg Slobodskoi",

rollup.config.js

@@ -135,5 +135,22 @@ export default [
       replace({'process.env.VERSION': JSON.stringify(pkg.version)}),
       sizeSnapshot(snapshotOptions)
     ]
+  },
+
+  {
+    input,
+    output: {file: pkg.unpkg, format: 'esm'},
+    plugins: [
+      nodeResolve(),
+      babel(getBabelOptions({useESModules: true})),
+      commonjs(commonjsOptions),
+      nodeGlobals({process: false}),
+      replace({
+        'process.env.VERSION': JSON.stringify(pkg.version)
+      })
+      // sizeSnapshot barfs with
+      // [!] (size-snapshot plugin) Error: Error transforming bundle with 'size-snapshot' plugin: ModuleNotFoundError: Module not found: Error: Can't resolve 'jss/node_modules/rollup-plugin-size-snapshot/node_modules/webpack/buildin/harmony-module.js' in '/'
+      // sizeSnapshot(snapshotOptions),
+    ]
   }
 ]