Add new keyboard shortcuts for Top/Bottom of page

We currently show available keyboard shortcuts when pressing `?` on keyboard:

![shortcuts](https://user-images.githubusercontent.com/466765/35009211-c4ea2600-fb63-11e7-995e-a04f1812bcee.png)

It would be useful to also add two commonly used keys:

* `Home` to go to top of the page (`Fn + Left arrow` on Mac)
* `End` to go to bottom of the page (`Fn + Right arrow` on Mac)

