enable providing custom fs-api

this would make the following possible:

```
var db = require('level')('/tmp/file.db')
var fs = require('level-filesystem')(db)
var send = require('send')
var http = require('http')
http.createServer(function(req, res){
  send(req, req.url, {fs:fs}).pipe(res)
}).listen(1337)
```