* 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
Renée Kooi