No syntax highlighting until first edit

[ncfavier]

Reopens #242.

In 0.6.4, write the following into test.lagda.md:

```agda
module _ where
```

Load the file. Duplicate the module _ where line:

```agda
module _ where
module _ where
```

Reload the file. The second line is not highlighted:

Make any edit. The second line becomes highlighted.