fixes line ending squashing

## steps

00.md:

```
Someone said:

> This is good.
```

## problem

Currently it renders:

Someone said: > This is good.

## what this changes

It's rendered:

Someone said:

> This is good.

## notes

This was introduced in 288e78ab1f2dd4535f94cbaa5216311f8cc29383.
I'm changing `source.getLines().mkString("")` back to `source.mkString("")`.

/cc @grahamar