Add support for common presentational attributes on feed tag

In order to save code and fit existing standard Textpattern tags, it would be useful to support at least the following attributes on the `feed` tag:
- `break` - HTML tag used in between items in the feed
- `wraptag` - HTML tag used to wrap the output of the block
- `class` - HTML class attribute to apply to `wraptag`
- `html_id` - HTML id attribute to apply to `wraptag`

It would also be useful to support the following:
- `breakby` - To group items
- `label` - Text to place before the output of the feed
- `labeltag` - HTML tag used to encapsulate the label text in `label`

The implementation should behave the same way as `` for example, with the exception that the defaults should be "unset" (for `breakby` this would mean a value of `1`).