Let admin specify which tags to allow

In some cases the anchor tag is too restrictive for the 'Allow tags' value.

Maybe it should at least match the core #description FAPI functionality (which I believe uses the 'filtered_html' text format, by default -- I haven't investigated this deeply).

Another simple way to handle this would be to expose the list of tags as a configurable variable extra_field_description_tags that would default to just <a>, and one could override the value in settings.php.