Tags don't get interpolated: "{author.name}", "{title}".

I run ytdl as:

```
$ ytdl -o "{author.name} - {title}"
```

and resulting file gets a name of `'{author.name} - {title}.mp4'` so no interpolation takes place.

I thought that maybe props are empty, but here is what info command (`-i `) shows:

```
$ -q 247 -i
title: My Trip From ...
author: Coac...
avg rating: undefined
...
```

Maybe `{author.name}` was wrong and I should have used `{author}` instead, but title was not substituted as well.

```
$ ytdl --version
1.4.1
```