Added command to run benchmarks with some old commits

Just to start a discussion on this - I think it would be nice when we add new benchmarks to run them with some of the old commits. The current PR will actually make it run commits that have never been benchmarked before (which also adds points for existing benchmarks) but it might be nicer if we could run new benchmarks with already benchmarked committed, to 'complete' them. Is this what ``EXISTING`` will do? If so, maybe we could do both:

```
asv run EXISTING --steps=10
asv run MISSING --steps=10
```

@mdboom - what do you think is the right approach?