Ignore tidy errors

When tidy finds errors, it produces no output at all. In this case, keep
the untidy entry body. This is better than no body at all.
Allow to adjust the entries’ HTML heading levels

There are feeds that publish the entries in a page context, i.e. they
contain headings that don't fit the layout of the aggregated page
produced by plagg and blosxom. This feature allows to adjust these
headings.