Support for WordPress export format namespace

What steps will reproduce the problem?

1. Take any exported RSS feed from a WordPress blog (Admin > Tools > Export)
2. Run it through feedparser
3. Try and extract custom WordPress tags (prefixed 'wp:') that appear to 
'clash' with feedparser (even though they are prefixed/namespaced with 'wp:'). 
Examples that are ignored (tags that also have child elements):

<wp:author>
<wp:category>

Examples that are parsed correctly:

<wp:wxr_version>
<wp:base_site_url>
<wp:base_blog_url>

What is the expected output? What do you see instead?

I would expect to see:

feed.wp_author = []
feed.wp_author[i].author_id = 1
feed.wp_author[i].author_login = 'admin'
feed.wp_author[i].author_email = 'foo@bar.com'
feed.wp_author[i].author_display_name = 'Foo Bar'
feed.wp_author[i].author_first_name = 'Foo'
feed.wp_author[i].author_last_name = 'Bar'

What version of the product are you using? On what operating system?

feedparser 5.1.3
Python 2.7.5
Mac OSX 10.9.3

Please provide any additional information below.

If this is beyond the scope of feedparser (ie. Word Press is not considered a 
valid RSS feed format) then please have some generic documentation on the 
feedparser site that lists how to monkey patch the feedparser code. In 
particular, how to deal with entries that need to have lists, such as the 
wp_author element listed in this issue.

Thanks in advance.

Original issue reported on code.google.com by robertln...@gmail.com on 13 Jun 2014 at 5:01

Start by reproducing the issue with a WordPress Admin > Tools > Export RSS feed using feedparser 5.1.3, then inspect the current handling of namespaced elements and child elements. Done means wp:author and wp:category are exposed with their nested fields, or the documentation explains how to register equivalent list-valued custom elements.