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

README.md

@@ -459,9 +459,18 @@ See [#routing](#routing) for an overview of how to use routing efficiently.
 Start the application and mount it on the given `querySelector`,
 the given selector can be a String or a DOM element.
 
-This will _replace_ the selector provided with the tree returned from `app.start()`.
+In the browser, this will _replace_ the selector provided with the tree returned from `app.start()`.
 If you want to add the app as a child to an element, use `app.start()` to obtain the tree and manually append it.
 
+On the server, this will save the `selector` on the app instance.
+When doing server side rendering, you can then check the `app.selector` property to see where the render result should be inserted.
+
+Returns `this`, so you can easily export the application for server side rendering:
+
+```js
+module.exports = app.mount('body')
+```
+
 ### `tree = app.start()`
 Start the application. Returns a tree of DOM nodes that can be mounted using
 `document.body.appendChild()`.

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