Make express.static use the "etag" application setting

I saw this feature on the 5.0 roadmap #2237 and I initially thought that it would need to be implemented in the serve-static module (https://github.com/expressjs/serve-static/pull/64). I think maybe I had misunderstood the use case as the comments I received there were that the changes needed to be made in express.

So after getting it wrong the first time I wanted to open this issue as a discussion item around the requirements for this feature and how best to implement it.

I think now the use case is this:

``` js
var express = require('express');
var app = express();
app.disable('etag');
app.use(express.static('/static')); // also disables etag without needing {'etag':false}
```

So if etags are disabled **before** express.static is called then they are also disabled for static content.

Does this make sense? Any comments appreciated.