#3 Support multiple, merged feeds

[phil]

Rome supports the capability to create a single RSS feed as an amalgamation of several independent source feeds. rss4jsp should be extended to provide this capability through its tags.

Options:
- Extend the existing feed tag
- Create a new tag that accepts multiple feeds.

There should be two supported ways of supplying the list of feeds:
- As an array or collection of String objects in the request, session or application scope
- As one or more nested tags, for example:

<feed>
<source url="..." />
<source url="..." />
</feed>