use default highlight language if none was given

When using the `literalinclude` directive inside a configuration
block, it's a tedious task to always pass the language used to
highlight the included code file like this:

```
.. configuration-block::

    .. literalinclude:: /foo.php
        :language: php
```

Now, you can omit the `language` to let the file being rendered with
the language configured by the `highlight_language` option:

```
.. configuration-block::

    .. literalinclude:: /foo.php
```

This way, a `literalinclude` directive used inside a configuration
block works in the same manner as outside of it.