Add minimaps (overview maps)

[joewdavies]

Something like this:

(credit: geoviz)

API could either be integrated into the existing map.insets([]) map instances or be something new like

map.minimap({
    x:400, 
    y:400, 
    position:{x,y,z}, 
    projection: d3.geoOrthographic().rotate([-100, -30])
})

Probably best to expand the existing map.insets() capabilities.