Suggested changes to rss_entry in  feedgen/entry.py

When a `content` attribute is given but there is no `description`, the argument to `content` is written to xml inside `description` tags.

Using the example in `feedgen/__main__.py`, something like `fe.content("Lorem ipsum dolor sit amet, consectetur adipiscing elit.")` would turn into `<description>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</description>.`

I'm not sure if a `description` tag is required for it to be valid xml or whether it was overlooked, but I think it would be nice to have the option.  Especially when the user specifically requests that the data be written as `content`!