Official geojson types

Lots of people use mongoose with geojson, so it would be handy to have officially supported types so you can do things like:

```javascript
new schema({
poly: mongoose.Types.Polygon,
feature: mongoose.Types.Feature(mongoose.Types.Polygon)
});
```

Re: #6599, #6597

Start by reviewing related issues #6599 and #6597, then locate how Mongoose currently defines schema types. Done means officially supported Polygon and Feature types can be used in schemas as shown in the issue.

Written by the indexing model from the issue text.