Add GetFeatureInfo format

This PR proposes to add a format reader for WMS GetFeatureInfo request.

The format is able to read WMS GetFeatureInfo responses that starts with `<msGMLOutput>` or `<FeatureCollection>`.

It is based on `ol.format.GML2` format to read the features.
This has a side effect. Don't you want to revert this in an `after` hook?