Multiple selection

This is a first implementation of multiple selection that kind of works on the WorldBank data viz.

So far by default the time variable is single selection and all other variables are multiple selection. That needs to be configurable at compile time.

Some multiple selection updates are not instantaneaous -- why?

Also the lines sometimes update and sometimes do not -- why?

http://bl.ocks.org/tdhock/raw/b1a59324286448047961/
I agree backwards compatibility is important, but I think multiple selection is more natural and will preferred in many cases. As for a heuristic, maybe a decent approach is to check whether a showSelected aesthetic is combined with a constant x/y aesthetic. If it is, you probably want single selection. For example, in the last graphic of [my PITCHf/x animation example](http://cpsievert.github.io/baseballR/20140818/), I label the speed of the pitch at the top of the plot. This doesn't work great when selecting multiple pitches. In this case, a [constant x/y aesthetic is used to map the text](https://github.com/cpsievert/cpsievert.github.com/blame/5116b3f3668c3416af189f2b91ea9435a9b689c6/baseballR/20140818/index.Rmd#L141-143)