Change GeoJSON geometry when tag suggests area feature

[vgeorge]

See #128.

@geohacker I suggest testing with preset "Apartment Complex".

[geohacker]

@vgeorge this looks great, thank you! One thing we should probably confirm: when a feature is considered to be an area, do we now add the area=yes tag? Does iD do that? If it does, we should add that to the properties.

[vgeorge]

@geohacker, we don't. I did two tests in iD: added a feature by selecting "Area" button at the top and selected a "area" preset on a closed way, none of them added area=yes. If I apply this tag manually to a feature in iD, it will be rendered differently, but presets doesn't seem to change it. It seems it uses area=* exclusively for rendering purposes.