Add support for common presentational attributes on feed tag

[AshleyButcher]

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 <txp:category_list /> for example, with the exception that the defaults should be "unset" (for breakby this would mean a value of 1).