[scrollycoding] Preserve files from previous steps

[pomber]

Instead of repeating the same file for each step:

<Hike>

<StepHead>

```js somefile.js
console.log(1)
console.log(2)
```

</StepHead>

Markdown for step 1

<StepHead focus="1">

```js somefile.js
console.log(1)
console.log(2)
```

</StepHead>

Markdown for step 2

</Hike>

Use the files defined in the previous step and only override the ones defined in the current step.

<Hike>

<StepHead>

```js somefile.js
console.log(1)
console.log(2)
```

</StepHead>

Markdown for step 1

<StepHead focus="1" />

Markdown for step 2

</Hike>

[pomber]

Fixed in 91fcf3e

[rachelnabors]

This is great!