In verbose mode, show information about included config files

When running in verbose mode, you get `user configuration: .config/beets/config.yaml`. It would be nice to have information about all included files with `config.html#include` directive. Example:

You have in config.yaml:

```
include: ['tokens.yaml', '~/external.yaml']
```

Running with verbose on would give:

`user configuration: .config/beets/config.yaml, .config/beets/tokens.yaml, ~/external.yaml`