Express disk cache

Middleware for caching responses sent through Express to disk.

#### Why does this exist?

Because web servers are good at delivering static files. It's also useful if you have NGINX sitting in front of your node.js webserver and want to take some of the load off it.

This also makes cache invalidation quite simple: just delete the file.

#### Install

Use your package manager of choice, for example:

``
npm install express-disk-cache
`

#### API

Then include the middleware in your Express app:

`javascript
var cache = require('express-disk-cache');
`

#### Example

`javascript
var app = Express();

// This will cache every response to disk
// assuming it meets the conditions described below
app.use(cache({
directory: __dirname + '/cache'
}));
``

#### How it works

As I understand it, HTTP response in express and node is a writeable stream. It wraps the res.write and res.end functions so it will work no matter how you send the response, using res.send, res.render, res.sendStatus, res.sendFile etc...

#### Conditions for caching

- The cache-control header is not set to no-cache
- There is no query string in the URL
- There is no session associated with the request

#### To do

Check that the URL contains valid characters that we can use to create a file on the file system.
Re-implement the filter function used by compression.