[DO NOT MERGE] env vars, what about them?

@julian and I had a chat about that, NodeBB should probably honor `process.env.VARS` better. 
this issue came up here on [this c9 thread](http://community.nodebb.org/topic/599)

one suggestion was to "prepend" every value in `config.json` with `env.` then NodeBB will load it from `process.env` instead. 
 i.e
```
{ 
 "port": "env.PORT",
 "bind_address": "env.IP",
 "database": "redis"
```

one downside to that is that `nconf.file()` won't work anymore, and I personally don't like this since I've used that function in multiple places and I like it :D