Added parametrization of rss feed names.

This PR adds the possibility to parametrize the rss feed name. 
As it turns out, sometimes you need to call your rss something other than "rss.xml", I for instance use - and communicated "feed.xml" to other aggregators.
If you specifiy ":rss-name" in the config.edn file, say "feed.xml", your feed will be found at this address. If you put nothing, this will fall back to "rss.xml".
Please do test this functionality, I was not able to do so as I did not grab how to do to test leiningen plugins...