Idea: make <main> optional

From `export.rs`:

```rust
            Event::Enter(Container::Document(_)) => self.output += "<main>",
            Event::Leave(Container::Document(_)) => self.output += "</main>",
```

I would like to see that as a configuration option. Sometimes, one wants to parse more than one `org` file for an HTML page, and more than one `<main>` element is not something that should be done.