Add support for images in RSS <item>

[jacobwhall]

RSS supports tags, that can include multimedia such as pictures. Especially with some of my photos being only images, it would be better if people can see the image in their reader without clicking through to the post.

This will require the database storing MIME types for each file. This will be important to keep in mind when designing a photo/file storage system.

[jacobwhall]

Now that I'm using splitflap, I'll have to check out how that library handles feed-items with multimedia.