map properties: ability to select start and end zoom level

[bartvde]

I think it might also be worth expanding the 'number of zoom levels' to a zoom slider where map makers can select the start and end zoom levels, not just the total number of levels. I feel like we want map makers to have control over this - you want users to be able to zoom in and check things out, but going down to zoom level 18 on a map showing country GDP seems a bit silly. This should also save us from generating a lot of tiles that aren't that useful. I think Rollie has some wireframes somewhere on this, a bar with multiple sliders to set the min and max.