Add Ace template engine support

This feature is discussed at forum topic "[Amber templates not working… documentation?](http://discuss.gohugo.io/t/amber-templates-not-working-documentation/129/15)". The needed upstream (Ace) modification was merged so it's time to send this.

This detects `.ace` suffix of a template file and treat it as a template written in Ace. It has a small trick

```
name = name[:len(name)-len(ext)] + ".html"
```

to avoid a layout call error (like "Unable to locate layout") at rendering pages because `.html` suffix is hard coded in many caller functions. It also has unexpected feature using multi template engine at the same time.

It only affects an internal template name, not a real file name but I think it's better to use a template engine independent name in caller functions or allow to specify a suffix it in future version.