Allow custom attribute names

Trying to set an attribute name dynamically (eg. `button (htmx_method)=(htmx_path)`) returns the error:

```
element body must be wrapped in braces

  = help: see https://github.com/lambda-fairy/maud/pull/137 for details
```

For custom libraries like HTMX, having the ability to set attributes dynamically would be very useful for type safety.