Make run button disappear after clicking.
Allow coliru-specific formatting.
Update output area async while running.
Make button a <div> and delegate styling to user.

They can make it appear as a button again using:

```css
.coliru .runbutton {
    appearance: button;
    -webkit-appearance: button;
    -moz-appearance: button;
}
```
Only add run button to <code> elements inside a <pre> element.

Also hoist our output area out of the original <pre> and into its own.  Allows us to adopt separate formatting.