server: Store mount selector and return this (#625)

* Store mount selector and return `this`

This allows `.mount('body')` to be used on the server like so:

```js
module.exports = app.mount('body')
```

Then, when server rendering you can do:

```js
var app = require('./app')
fillServerTemplate(html, app.selector, app.toString('/'))
```

Where `fillServerTemplate` is a function that replaces the HTML at a
CSS selector with some string.

This way you do not need to check for `typeof window` in your
application entry point to do server rendering, and it allows tools like
bankai to automatically inline server rendered html in the correct place.

* docs

* better words

index.js

@@ -153,7 +153,12 @@ Choo.prototype.start = function () {
 }
 
 Choo.prototype.mount = function mount (selector) {
-  assert.equal(typeof window, 'object', 'choo.mount: window was not found. .mount() must be called in a browser, use .toString() if running in Node')
+  if (typeof window !== 'object') {
+    assert.ok(typeof selector === 'string', 'choo.mount: selector should be type String')
+    this.selector = selector
+    return this
+  }
+
   assert.ok(typeof selector === 'string' || typeof selector === 'object', 'choo.mount: selector should be type String or HTMLElement')
 
   var self = this