Check that the `four_space_rule` extension works for plain writer.

```
% pandoc -f markdown -t plain+four_space_rule
This is the title

Here we fix:

- a
- b
- c
^D
This is the title

Here we fix:

-   a
-   b
-   c
```

Check that the `four_space_rule` extension is off by default.

```
% pandoc -f markdown -t plain
This is the title

Here we fix:

- a
- b
- c
^D
This is the title

Here we fix:

- a
- b
- c
```
