let elpher handle gemini, gopher links in eww buffer

In a eww buffer, clicking on gemini, gopher, finger links will give
error "Unknown url scheme" as eww would try to open it itself.

Eww calls browse-url if a link matches regexp as defined in
`eww-use-browse-url' variable. Elpher has already defined
`elpher-browse-url-elpher' handler to handle gemini links, so browse-url
would open those links in elpher appropriately.