Add benchmarking support

[sethkinast]

It's probably best if we use a shootout engine that already exists somewhere, and make it part of a testing suite. That keeps us from having to dep a bunch of engines as devdeps or include them.

Possible candidates that I know of are Handlebars' benchwarmer suite, and https://github.com/raptorjs/templating-benchmarks

[sethkinast]

This is primarily for us to catch perf regressions between versions. But never hurts to compare yourself against Jade to make yourself feel better.