Continuous integration of benchmarks

[lelemmen]

Describe the feature you'd like
It would be nice to use the benchmarks as a criterion for CI, for example PRs may not be merged if the benchmarks are slower by for example 15% or so.

Additional context
We should of course come up with meaningful benchmarks, in a meaningful isolated environment. We could also provide the most up-to-date benchmarks (and the specifications of the system on which they were run) on our website or one of the informative files of this repository.

@guacke @nbilliet something interesting to look at?