Make feed's body configurable

[eelcoj]

Both RSS/XML and Json feed body's are hard-coded. They provide sane/conventional/standard output, but like to provide an option to override with your own template.

As a first step, moving the body into an erb-template seems to be the best step, while keeping test pass. Second option is then to add a “seam” where developer can add their own either by using the same look up path for the template or passing a file path as an option.