Allow map to be re-rendered with a column sort

[mrjones-plip]

We should allow a column, when clicked to change it's sort, to have the map re-render so that the choropleth values are derived from the currently sorted column. So if we have a CSV file like this:

latitude	longitude	country	foo	bar	note
1.2	2.8	US	29	4000	banana
4.9	9.0	MX	10	5000	apple

If the map was initially rendered using the default value for countries.attr.rollup (length), when you click column foo, it would have the choropleth values take from that column instead of length.

This feature would need to be off by default so that you couldn't click note, for example. Also, we'll need to have a custom rollup() function per column so that MapTable can be customized on how to render the new map. Oh...hmm...maybe we could get away with a shared one by default? Let's see!

The URL should be stateful such that the current data source for the map is preserved and can be restored when rendering a URL. We can fix #69 when we do this ticket!

[mrjones-plip]

about to release 1.7.0, closing this as one of the primary features in that release.