CI performance benchmarks

Agreed with https://github.com/dignifiedquire/pull-block/pull/11#issuecomment-343753577

> Any chance you could put the benches in a format we can run them on ci? That way we could make sure we don’t regress

That would be the way to go. Though not having as much time to figure this out as I would like, here's an issue to track the progress on that

Over at https://github.com/dignifiedquire/pull-block/issues/3#issuecomment-331101267

> There is a nice setup for this which compares current checked out version with the npm version here for example: https://github.com/crypto-browserify/browserify-aes/blob/master/bench/index.js

Perhaps @dignifiedquire can set up a similar test with aegir to provide the framework.
And I could expand it with a selection of the ~90 different test parameters I worked with.