`details` directive doesn't respect `:class:` option

[2bndy5]

I noticed that the directive only adds classes inherent to the container nodes. I might be misdiagnosing the problem here.

https://github.com/tk0miya/sphinxcontrib-details-directive/blob/f7cd07543718dfd79b22451f4d2bac71792ad03f/sphinxcontrib/details/directive/__init__.py#L55-L59

Locally I was able to fix this using list concatenation:

    def run(self):
        admonition = nodes.container(
            "",
            classes=self.options.get("class", []) + self.options.get("classes", []),
            opened="open" in self.options,
            type="details",
        )

**fix was formatted with black to avoid a long line

[2bndy5]

@tk0miya I can submit a PR. We have been monkeypatching this extension with the suggested fix, and it works well.

We recently noticed the transform doesn't support the dirhtml builder, so my PR would also include that fix as well.