Strip a single leading space from bird tracks (#201)

This makes bird tracks in the form

```
> foo
> bar
> bat
```

parse as if they had been written as

```
>foo
>bar
>bat
```

ie. without the leading whitespace in front of every line.

Ideally we also want to look into how leading whitespace affects code
blocks written using the @ @ syntax, which are currently unaffected by
this patch.