Add feed functions to generate feeds with external links

We ran into a situation yesterday where we needed to generate a feed of
external links (so `Text`, not `Route m`). The Feed types are parameterized, so
they support it, but the generation functions require routes and use
`getUrlRenderer` to render them.

The simplest fix was to duplicate the generation functions as versions that
take `Feed Text` and use `id` as the renderer.

Another option could be to use a type class, but I thought this easy,
backwards-compatible addition was a good start.

*Note*: I just pulled these changes in from some in-project hacks but don't
have the time to compile/test this PR right now. I'm hoping that travis builds
PRs so I may not have to; if that's not the case, I'll report back later when I
do get the time to ensure there are no typos here, etc.