Expose more loopback middleware for require

/to @bajtos 

We need a way to reference loopback.favicon and other middleware from middleware.json. 
`favicon` is different from `errorhandler` because loopback adds a default favicon file. I can't think of a better way how to provide the default favicon, thus we have to expose the middleware this way.

Few things to consider:

 - Should the file be called `loopback/middlewarwe/serve-favicon.js`? That is more consistent with `serve-favicon`.
 - The usual practice is to put the meat of the implementation in the per-feature file (`favicon.js`) and keep the index file (`express-middleware.js`) just a lightweight aggregator. We already have a mechanism in place that loads all middleware from `server/middleware/*.js`. Can you move all of `favicon` implementation to this new file?