improve the config generator

[gildesmarais]

The feed config generator, in its current state, is a bit clunky to use. The rails world is full of generators which scaffold some files. So, I imagine, having rake config https://example.com/path could automatically:

• create the feed config
• create the feed config's spec
• could prefill some values in the feed config (channel title (from page's <title>)), language, etc (open graph and meta tags)

In another iteration it could also suggest the selectors, but that's a topic for another day.