font-feature-settings improvements

Follows-up discussion https://github.com/mapnik/mapnik/pull/2416#issuecomment-58825489

Things to do:

- [x] Apply `font-feature-settings` per `text_item`. This allows formatting like "H₂SO₄":

```
<TextSymbolizer>
    "H"<Format font-feature-settings="subs 1">"2"</Format>"SO"<Format font-feature-settings="subs 1">"4"</Format>
</TextSymbolizer>
```

- [x] Find out why most of the features are not working with fonts provided with Mapnik. These fonts most likely don't support OpenType features well. I will try if some other fonts will work better.

- [x] Improve test coverage: per `Format` node setting, all fonts provided with Mapnik, more features.