[wip] [dev] Disallow localhost as backend

[EverlastingBugstopper]

Fixes #902

$ wrangler dev -h http://localhost:8000
💁  JavaScript project found. Skipping unnecessary build!
Error: You cannot currently use wrangler dev with a local server as the host. You could use TryCloudflare to generate a public URL that you could then pass as the host to wrangler dev.
See https://developers.cloudflare.com/argo-tunnel/trycloudflare/ for more info

This approach is a bit naive at the moment, there are other endpoints that the preview service will return a rick roll embed on.

[gabbifish]

If we need to do this, I think we should be at parity with what capn proto checks for in restrict_peers(); just checking for localhost isn't enough.