Expose more loopback middleware for require

/to @bajtos 

We need a way to reference loopback.favicon and other middleware from middleware.json. 
> If it's agreed, we can simply reference it as loopback#favicon without adding extra JS files.

The current way of exposing the middleware via `safeRequire` is subtly broken - see #833.

While I can see that referencing exported middleware may make sense in general sense, I think it is a bad solution in this particular case. Unless we add `serve-favicon` to loopback's dependencies.