Allow Ghost to be used as middleware within another Express app.

closes #827
- accepts new key in config file 'middleware', including it with the value
true allows the 'server' key to be omitted.
- don't .listen() on the rootApp express instance when middleware:true is in config
- accepts new key in config file 'generate404s'.  When true or absent, Ghost
behaves normally.  With 'generate404s:false', don't serve the 404 page.
- don't produce the Ghost-no-longer-auto-starts warning when 'middleware:true'

Note:  wiki page that can be copied into Ghost's wiki at
https://github.com/gleneivey/Ghost/wiki/Using-Ghost-as-Express-Middleware
And repo with demo Express app at https://github.com/gleneivey/example-express-with-ghost-middleware