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:

```javascript
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 ".".

I ran into this issue when trying to extract the configuration for the default view extension name into a config file and represent it as a variable when configuring my Express app. I realized that an app's `engine()` registration and "view engine" setting do not behave the same; this give them parity.