Expose more loopback middleware for require

/to @bajtos 

We need a way to reference loopback.favicon and other middleware from middleware.json. 
I'm following the same pattern as `static`. 

See my comment at https://github.com/strongloop/loopback-boot/pull/70#issuecomment-63668278. If it's agreed, we can simply reference it as `loopback#favicon` without adding extra JS files.