Recipe for elfeed-web

This is an AngularJS web interface for Elfeed, an Emacs web feed (RSS, Atom) reader. This PR depends on the skeeto/elfeed recipe PR since it's useless without that recipe.

https://github.com/skeeto/elfeed

This is distributed separately because I don't want the core package to depend on the web server package.

I've tested that the package builds and installs properly.