Add trim_namespace argument for app.config.get_namespace (with tests)

This pull request is identical with #1184. I only added tests for it. 

@untitaker, you also are asked to add entry to changelog, but the method `get_namespace` was first introduced in version 1.0:

```
- Added :meth:`flask.config.Config.get_namespace`.
```

I think that adding another entry in the same list does not make sense.