Allow virtual dispatch from runners

[gadomski]

Right now a runner can't be given a generic "transform", since all of our dispatch is done at compile-time via the runner template class. While I'd like to keep the compile-time dispatch, we should add virtual dispatch as well, for the case of downstreams that want to reuse the same runner configuration for rigid and nonrigid (e.g.).

As I'm writing this, I'm wondering if #81 might be good enough...so I should probably do #81 first before this one.