allow seed to be in a separate file

[Shaun Hamilton]

The project files can get massive very quickly. Perhaps we want to be able to separate out the ### --seed-- into a markdown file of its own.

We could have:

<!-- learn-x-by-building-y.md -->

# Learn X by Building Y

## 1

### --description--

Some description...

### --tests--

Some test...

```js
// Test
```

## 2

...

<!-- any-file-name-seed.md -->
# Learn X by Building Y

## 1

### --seed--

#### --cmd--

```bash
touch example.js
```

## 2

### --seed--

#### --"example.js"--

```js
const a = 1;
```

...