Add full extname support to the "view engine" setting; e.g., ".jade".

This allows View to support a `defaultEngine` (a.k.a. an app's
"view engine" setting) which contains a ".", for example:

```
app.engine('.jade', jadeEngine);
app.set('view engine', '.jade');
```

This brings View's handling of template filename extensions to parity
with `app.engine()`.

This allows an app's "view engine" setting to be a full extension name,
including the ".".
Merge pull request #1466 from ericf/settings-view-engine

Add full extname support to the "view engine" setting; e.g., ".jade".