Support include/exclude URIs for enforcing timeout

Added feature to support list of URIs that can be included/excluded for Rack::Timeout

Included URIs can be specified like this:

```
Rack::Timeout.include_uris = ['foo']
```

Excluded URIs can be specified like this:

```
Rack::Timeout.exclude_uris = ['bar']
```

If no included URIs are specified, timeout is enforced on all requests.